SPI 1275 Vanguard Extended Duration Treasury Index Fund

Vanguard Extended Duration Treasury Index Fund

Summary Prospectus

December 20, 2024

Institutional Shares & Institutional Plus Shares

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

The Fund’s statutory Prospectus and Statement of Additional Information dated December 20, 2024, 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 and https://personal.vanguard.com/us/literature/reports/MFs. 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.

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.

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 tables describe 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 tables and examples 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	0.05 %	0.04%
12b-1 Distribution Fee	None	None
Other Expenses	0.01 %	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 22% 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 U.S. Treasury STRIPS 20–30 Year Equal Par Bond Index (the Index). The 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. A Treasury 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, 2024, the dollar-weighted average maturity of the Index was 24.5 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-related risks. The Fund is subject to risks associated with index investing, which include passive management risk, tracking error risk, and index provider risk. Passive management risk is the chance that the Fund’s use of an indexing strategy will negatively impact the Fund’s performance. Because the Fund seeks to track the performance of its target index regardless of how that index is performing, the Fund’s performance may be lower than it would be if the Fund were actively managed. Tracking error risk is the chance that the Fund’s performance will deviate from the performance of its target index. Tracking error risk may be heightened during times of increased market volatility or under other unusual market conditions. Index provider risk is the chance that the Fund will be negatively impacted by changes or errors made by the index provider. Any gains, losses, or costs associated with or resulting from an error made by the index provider will generally be borne by the Fund and, as a result, the Fund’s shareholders.

• 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.

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

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2024, was -0.13%.

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

	Total Return	Quarter
Highest	29.71 %	March 31, 2020
Lowest	-18.89 %	September 30, 2023

Average Annual Total Returns for Periods Ended December 31, 2023

	1 Year	5 Years	10 Years
Vanguard Extended Duration Treasury Index Fund Institutional Shares
Return Before Taxes	1.58 %	-3.16 %	2.81 %
Return After Taxes on Distributions	0.12	-4.41	1.40
Return After Taxes on Distributions and Sale of Fund Shares	0.91	-2.62	1.78
Vanguard Extended Duration Treasury Index Fund Institutional Plus Shares
Return Before Taxes	1.61 %	-3.14 %	2.83 %
Bloomberg U.S. Treasury STRIPS 20-30 Year Equal Par Bond Index (reflects no deduction for fees, expenses, or taxes)	1.09 %	-2.94 %	2.81 %
Bloomberg U.S. Aggregate Float Adjusted Index (reflects no deduction for fees, expenses, or taxes)	5.60	1.17	1.83

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 982901, El Paso, TX 79998-2901), 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 gains. 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. You should consult your own tax advisor with respect to any particular U.S. or non-U.S. tax consequences of your investment in the Fund.

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®” and the Bloomberg U.S. Treasury STRIPS 20-30 Year Equal Par Bond Index and Bloomberg U.S. Aggregate Float Adjusted Index (the “Indices”) are service marks of Bloomberg Finance L.P. and its affiliates, including Bloomberg Index Services Limited (“BISL”), the administrator of the index (collectively, “Bloomberg”), and have been licensed for use for certain purposes by Vanguard.

Vanguard Extended Duration Treasury Index Fund (the “Fund”) is not sponsored, endorsed, sold or promoted by Bloomberg. Bloomberg does not make any representation or warranty, express or implied, to the owners of or counterparties to the Fund or any member of the public regarding the advisability of investing in securities or commodities generally or in the Fund particularly. The only relationship of Bloomberg to Vanguard is the licensing of certain trademarks, trade names

and service marks and of the Indices, which are determined, composed and calculated by BISL without regard to Vanguard or the Fund. Bloomberg has no obligation to take the needs of Vanguard or the owners of the Fund into consideration in determining, composing or calculating the Indices. Bloomberg is not responsible for and has not participated in the determination of the timing, price, or quantities of the Fund to be issued. Bloomberg shall not have any obligation or liability, including, without limitation, to customers of the Fund, in connection with the administration, marketing or trading of the Fund.

BLOOMBERG DOES NOT GUARANTEE THE ACCURACY AND/OR THE COMPLETENESS OF THE INDICES OR ANY DATA RELATED THERETO AND SHALL HAVE NO LIABILITY FOR ANY ERRORS, OMISSIONS OR INTERRUPTIONS THEREIN. BLOOMBERG DOES NOT MAKE ANY WARRANTY, EXPRESS OR IMPLIED, AS TO RESULTS TO BE OBTAINED BY VANGUARD, OWNERS OF THE FUND OR ANY OTHER PERSON OR ENTITY FROM THE USE OF THE INDICES OR ANY DATA RELATED THERETO. BLOOMBERG DOES NOT MAKE ANY EXPRESS OR IMPLIED WARRANTIES AND EXPRESSLY DISCLAIMS ALL WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE OR USE WITH RESPECT TO THE INDICES OR ANY DATA RELATED THERETO. WITHOUT LIMITING ANY OF THE FOREGOING, TO THE MAXIMUM EXTENT ALLOWED BY LAW, BLOOMBERG, ITS LICENSORS, AND ITS AND THEIR RESPECTIVE EMPLOYEES, CONTRACTORS, AGENTS, SUPPLIERS, AND VENDORS SHALL HAVE NO LIABILITY OR RESPONSIBILITY WHATSOEVER FOR ANY INJURY OR DAMAGES—WHETHER DIRECT, INDIRECT, CONSEQUENTIAL, INCIDENTAL, PUNITIVE OR OTHERWISE—ARISING IN CONNECTION WITH THE FUND OR INDICES OR ANY DATA OR VALUES RELATING THERETO—WHETHER ARISING FROM THEIR NEGLIGENCE OR OTHERWISE, EVEN IF NOTIFIED OF THE POSSIBILITY THEREOF.

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).

Vanguard Marketing Corporation, Distributor.

SPI 1275 122024