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Vanguard Extended DurationTreasury Index Fund Summary Prospectus

December 22, 2020

Institutional Shares & Institutional Plus Shares

Vanguard Extended Duration Treasury Index Fund Institutional Shares (VEDTX) Vanguard Extended DurationTreasury Index Fund Institutional Plus Shares (VEDIX)

The Fund's statutory Prospectus and Statement of Additional Information dated December 22, 2020, as may be amended or supplemented, are incorporated into and made part of this Summary Prospectus by reference.

Before you invest, you may want to review the Fund's Prospectus, which contains more information about the Fund and its risks. You can find the Fund's Prospectus and other information about the Fund online at www.vanguard.com/prospectus. You can also obtain this information at no cost by calling 800-662-7447 (if you are an individual investor) or 888-809-8102 (if you are a client of Vanguard's Institutional Division) or by sending an email request to online@vanguard.com.

See the inside front cover for important information about access to your fund's annual and semiannual shareholder reports.

The Securities and Exchange Commission (SEC) has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

Important information about access to shareholder reports

Beginning on January 1, 2021, as permitted by regulations adopted by the SEC, paper copies of your fund's annual and semiannual shareholder reports will no longer be sent to you by mail, unless you specifically request them. Instead, you will be notified by mail each time a report is posted on the website and will be provided with a link to access the report.

If you have already elected to receive shareholder reports electronically, you will not be affected by this change and do not need to take any action. You may elect to receive shareholder reports and other communications from the fund electronically by contacting your financial intermediary (such as a broker-dealer or bank) or, if you invest directly with the fund, by calling Vanguard at one of the phone numbers on the back cover of this prospectus or by logging on to vanguard.com.

You may elect to receive paper copies of all future shareholder reports free of charge. If you invest through a financial intermediary, you can contact the intermediary to request that you continue to receive paper copies. If you invest directly with the fund, you can call Vanguard at one of the phone numbers on the back cover of this prospectus or log on to vanguard.com. Your election to receive paper copies will apply to all the funds you hold through an intermediary or directly with Vanguard.

Investment Objective

The Fund seeks to track the performance of an index of extended-duration zero-coupon U.S. Treasury securities.

Fees and Expenses

The following table describes the fees and expenses you may pay if you buy, hold, and sell Institutional Shares or Institutional Plus Shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.

Shareholder Fees

(Fees paid directly from your investment)

	Institutional Shares	Institutional Plus Shares
Sales Charge (Load) Imposed on Purchases	None	None
Purchase Fee	0.50%	0.50%
Sales Charge (Load) Imposed on Reinvested
Dividends	None	None

Redemption Fee	None	None

Annual Fund Operating Expenses

(Expenses that you pay each year as a percentage of the value of your investment)

	Institutional Shares	Institutional Plus Shares
Management Fees	0.06%	0.04%

12b-1 Distribution Fee	None	None

Other Expenses	0.00%	0.00%

Total Annual Fund Operating Expenses	0.06%	0.04%

Examples

The following examples are intended to help you compare the cost of investing in the Fund's Institutional Shares or Institutional Plus Shares with the cost of investing in other mutual funds. They illustrate the hypothetical expenses that you would incur over various periods if you were to invest $10,000 in the Fund's shares. These examples assume that the shares provide a return of 5% each year and that total annual fund operating expenses remain as stated in the preceding table. You would incur these hypothetical expenses whether or not you were to redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year	3 Years	5 Years	10 Years
Institutional Shares	$56	$69	$84	$127

Institutional Plus Shares	$54	$63	$73	$101

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in more taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the previous expense examples, reduce the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 17% of the average value of its portfolio.

Principal Investment Strategies

The Fund employs an indexing investment approach designed to track the performance of the Bloomberg Barclays U.S.Treasury STRIPS 20–30Year Equal Par Bond Index.This Index includes zero-coupon U.S.Treasury securities (Treasury STRIPS), which are backed by the full faith and credit of the U.S. government, with maturities ranging from 20 to 30 years. ATreasury STRIP represents a single coupon payment, or a single principal payment, from a U.S.Treasury security that has been "stripped" into separately tradable components.

The Fund invests by sampling the Index, meaning that it holds a range of securities that, in the aggregate, approximates the full Index in terms of key risk factors, including duration, cash flow, and other characteristics. All of the Fund's investments will be selected through the sampling process and, under normal circumstances, at least 80% of the Fund's assets will be invested in

U.S. Treasury securities held in the Index. The Fund maintains a dollar-weighted average maturity consistent with that of the Index. As of August 31, 2020, the dollar-weighted average maturity of the Index was 24.7 years.

Principal Risks

An investment in the Fund could lose money over short or long periods of time. You should expect the Fund's share price and total return to fluctuate within a wide range. The Fund is subject to the following risks, which could affect the Fund's performance, and the level of risk may vary based on market conditions:

•Interest rate risk, which is the chance that bond prices overall will decline because of rising interest rates. The prices of long-term bonds, such as those held by the Fund, are much more sensitive to interest rate changes than are the prices of short-term bonds. Interest rate risk is expected to be very high for the Fund because it invests primarily in zero-coupon long-term bonds, which have prices that are even more sensitive to interest rate changes than are coupon-bearing bonds of similar maturity. Because the Fund invests primarily in Treasury STRIPS with maturities ranging from 20 to 30 years, rising interest rates may cause the value of the Fund's investments to decline significantly.

•Index sampling risk, which is the chance that the securities selected for the Fund, in the aggregate, will not provide investment performance matching that of the Fund's target index. Index sampling risk for the Fund is expected to

be low.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Annual Total Returns

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund's Institutional Shares (including annual fund operating expenses but excluding shareholder fees) has varied from one calendar year to another over the periods shown. If applicable shareholder fees were reflected, returns would be less than those shown in the bar chart. The table shows how the average annual total returns of the share classes presented (including annual fund operating expenses and any applicable shareholder fees) compare with those of the Fund's

target index and another comparative index, which have investment characteristics similar to those of the Fund. Keep in mind that the Fund's past performance (before and after taxes) does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.

Annual Total Returns — Vanguard Extended Duration Treasury Index Fund Institutional Shares1

	2010	2011	2012	2013		2014	2015		2016	2017	2018		2019
80%		55.79
80%
60%						45.66


40%	10.28									13.52			19.16
40%
20%
			3.25						1.53

0%
-20%								–4.43				–3.49
-20%					–20.89
-40%					–20.89
-40%

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2020, was 28.75%.

During the periods shown in the bar chart, the highest and lowest returns for a calendar quarter were:

	Total Return			Quarter
Highest	52.05%		September 30, 2011
Lowest	-16.97%		December 31, 2016
Average Annual Total Returns for Periods Ended December 31, 2019
		1 Year	5 Years	10 Years
Vanguard Extended Duration Treasury Index Fund
Institutional Shares

Return Before Taxes		18.57%	4.74%	9.93%

Return After Taxes on Distributions		16.98	3.25	8.21

Return After Taxes on Distributions and Sale of Fund Shares		11.08	3.01	7.32

Comparative Indexes
(reflect no deduction for fees, expenses, or taxes)

Bloomberg Barclays U.S. Treasury STRIPS 20-30 Year Equal
Par Bond Index		20.13%	4.90%	10.08%

Bloomberg Barclays U.S. Aggregate Bond Index		8.72	3.05	3.75

			Since
			Inception
			(Aug. 28,
	1 Year	5 Years	2013)

Vanguard Extended Duration Treasury Index Fund
Institutional Plus Shares
Return Before Taxes	18.59%	4.76%	9.36%
Comparative Indexes
(reflect no deduction for fees, expenses, or taxes)

Bloomberg Barclays U.S. Treasury STRIPS 20-30 Year Equal
Par Bond Index	20.13%	4.90%	9.56%

Bloomberg Barclays U.S. Aggregate Bond Index	8.72	3.05	3.50

Actual after-tax returns depend on your tax situation and may differ from those shown in the preceding table. When after-tax returns are calculated, it is assumed that the shareholder was in the highest individual federal marginal income tax bracket at the time of each distribution of income or capital gains or upon redemption. State and local income taxes are not reflected in the calculations. Please note that after-tax returns are shown only for the Institutional Shares and may differ for each share class. After-tax returns are not relevant for a shareholder who holds fund shares in a tax-deferred account, such as an individual retirement account or a 401(k) plan. Also, figures captioned Return After Taxes on Distributions and Sale of Fund Shares may be higher than other figures for the same period if a capital loss occurs upon redemption and results in an assumed tax deduction for the shareholder.

Investment Advisor

The Vanguard Group, Inc. (Vanguard)

Portfolio Manager

Joshua C. Barrickman, CFA, Principal of Vanguard and co-head of Vanguard's Fixed Income Indexing Americas. He has managed the Fund since 2013.

Purchase and Sale of Fund Shares

You may purchase or redeem shares online through our website (vanguard.com), by mail (The Vanguard Group, P.O. Box 1110, Valley Forge, PA 19482-1110), or by telephone (800-662-2739). The minimum investment amount required to open and maintain a Fund account for Institutional Shares or Institutional Plus Shares is $5 million or $100 million, respectively. The minimum investment amount required to add to an existing Fund account is generally $1. Vanguard Institutional Advisory Services clients should contact Vanguard for information on special eligibility rules that may apply to them regarding Institutional Shares. If you are

investing through an employer-sponsored retirement or savings plan, your plan administrator or your benefits office can provide you with detailed information on how you can invest through your plan.

Tax Information

The Fund's distributions may be taxable as ordinary income or capital gain. If you are investing through a tax-advantaged account, such as an IRA or an employer-sponsored retirement or savings plan, special tax rules apply.

Payments to Financial Intermediaries

The Fund and its investment advisor do not pay financial intermediaries for sales of Fund shares.

CFA® is a registered trademark owned by CFA Institute.

BLOOMBERG is a trademark and service mark of Bloomberg Finance L.P. BARCLAYS is a trademark and service mark of Barclays Bank Plc, used under license. Bloomberg Finance L.P. and its affiliates, including Bloomberg Index Services Limited (BISL) (collectively, Bloomberg), or Bloomberg's licensors, own all proprietary rights in the Bloomberg Barclays U.S. Treasury STRIPS 20-30 Year Equal Par Bond Index and Bloomberg Barclays U.S. Aggregate Bond Index (the Indices or Bloomberg Barclays Indices).

Neither Barclays Bank Plc, Barclays Capital Inc., or any affiliate (collectively Barclays) or Bloomberg is the issuer or producer of the Extended Duration Treasury Index Fund and neither Bloomberg nor Barclays has any responsibilities, obligations or duties to investors in the Extended Duration Treasury Index Fund. The Indices are licensed for use by The Vanguard Group, Inc. (Vanguard) as the sponsor of the Extended Duration Treasury Index Fund. Bloomberg and Barclays' only relationship with Vanguard in respect to the Indices is the licensing of the Indices, which is determined, composed and calculated by BISL, or any successor thereto, without regard to the Issuer or the Extended Duration Treasury Index Fund or the owners of the Extended Duration Treasury Index Fund.

Additionally, Vanguard may for itself execute transaction(s) with Barclays in or relating to the Indices in connection with the Extended Duration Treasury Index Fund. Investors acquire the Extended Duration Treasury Index Fund from Vanguard and investors neither acquire any interest in the Indices nor enter into any relationship of any kind whatsoever with Bloomberg or Barclays upon making an investment in the Extended Duration Treasury Index Fund. The Extended Duration Treasury Index Fund is not sponsored, endorsed, sold or promoted by Bloomberg or Barclays. Neither Bloomberg nor Barclays makes any representation or warranty, express or implied regarding the advisability of investing in the Extended Duration Treasury Index Fund or the advisability of investing in securities generally or the ability of the Indices to track corresponding or relative market performance. Neither Bloomberg nor Barclays has passed on the legality or suitability of the Extended Duration Treasury Index Fund with respect to any person or entity. Neither Bloomberg nor Barclays is responsible for and has not participated in the determination of the timing of, prices at, or quantities of the Extended Duration Treasury Index Fund to be issued. Neither Bloomberg nor Barclays has any obligation to take the needs of the Issuer or the owners of the Extended Duration Treasury Index Fund or any other third party into consideration in determining, composing or calculating the Indices. Neither Bloomberg nor Barclays has any obligation or liability in connection with administration, marketing or trading of the Extended Duration Treasury Index Fund.

The licensing agreement between Bloomberg and Barclays is solely for the benefit of Bloomberg and Barclays and not for the benefit of the owners of the Extended Duration Treasury Index Fund, investors or other third parties. In addition, the licensing agreement between Vanguard and Bloomberg is solely for the benefit of Vanguard and Bloomberg and not for the benefit of the owners of the Extended Duration Treasury Index Fund, investors or other third parties.

NEITHER BLOOMBERG NOR BARCLAYS SHALL HAVE ANY LIABILITY TO THE ISSUER, INVESTORS OR TO OTHER THIRD PARTIES FOR THE QUALITY, ACCURACY AND/OR COMPLETENESS OF THE BLOOMBERG BARCLAYS INDICES OR ANY DATA INCLUDED THEREIN OR FOR INTERRUPTIONS IN THE DELIVERY OF THE BLOOMBERG BARCLAYS INDICES. NEITHER BLOOMBERG NOR BARCLAYS MAKES ANY WARRANTY, EXPRESS OR IMPLIED, AS TO RESULTS TO BE OBTAINED BY THE ISSUER, THE INVESTORS OR ANY OTHER PERSON OR ENTITY FROM THE USE OF THE BLOOMBERG BARCLAYS INDICES OR ANY DATA INCLUDED THEREIN. NEITHER BLOOMBERG NOR BARCLAYS MAKES ANY EXPRESS OR IMPLIED WARRANTIES, AND EACH HEREBY EXPRESSLY DISCLAIMS ALL WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE OR USE WITH RESPECT TO THE BLOOMBERG BARCLAYS INDICES OR ANY DATA INCLUDED THEREIN. BLOOMBERG RESERVES THE RIGHT TO CHANGE THE METHODS OF CALCULATION OR PUBLICATION, OR TO CEASE THE CALCULATION OR PUBLICATION OF THE BLOOMBERG BARCLAYS INDICES, AND NEITHER BLOOMBERG NOR BARCLAYS SHALL BE LIABLE FOR ANY MISCALCULATION OF OR ANY INCORRECT, DELAYED OR INTERRUPTED PUBLICATION WITH RESPECT TO THE BLOOMBERG BARCLAYS INDICES. NEITHER BLOOMBERG NOR BARCLAYS SHALL BE LIABLE FOR ANY DAMAGES, INCLUDING, WITHOUT LIMITATION, ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES, OR ANY LOST PROFITS AND EVEN IF ADVISED OF THE POSSIBILITY OF SUCH, RESULTING FROM THE USE OF THE BLOOMBERG BARCLAYS INDICES OR ANY DATA INCLUDED THEREIN OR WITH RESPECT TO THE EXTENDED DURATION TREASURY INDEX FUND.

None of the information supplied by Bloomberg or Barclays and used in this publication may be reproduced in any manner without the prior written permission of both Bloomberg and Barclays Capital, the investment banking division of Barclays Bank Plc. Barclays Bank Plc is registered in England No. 1026167, registered office 1 Churchill Place London E14 5HP.

Vanguard Extended Duration Treasury Index Fund Institutional Shares—Fund Number 1275

Vanguard Extended Duration Treasury Index Fund Institutional Plus Shares—Fund Number 1276

To request additional information about the Fund, please visit vanguard.com or contact us at 800-662-7447 (if you are an individual investor) or 888-809-8102 (if you are a client of Vanguard's Institutional Division).

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