Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¹The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page. The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act.

CUSIP No. 928645 10 0

1)	Name of Reporting Person I.R.S. Identification No. of Above Person (Entities Only)	Domain Partners V, L.P.
2)	Check the Appropriate Box if a Member of a Group	(a) x (b) o
3)	SEC Use Only
4)	Citizenship or Place of Organization	Delaware
Number of Shares Beneficially Owned by Each Reporting Person With		5)	Sole Voting Power	3,885,309 shares of Common Stock
		6)	Shared Voting Power	-0-
		7)	Sole Dispositive Power	3,885,309 shares of Common Stock
		8)	Shared Dispositive Power	-0-
9)	Aggregate Amount Beneficially Owned by Each Reporting Person	3,885,309 shares of Common Stock
10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares
11)	Percent of Class Represented by Amount in Row (9)	8.3%
12)	Type of Reporting Person	PN

CUSIP No. 928645 10 0

1)	Name of Reporting Person I.R.S. Identification No. of Above Person (Entities Only)	DP V Associates, L.P.
2)	Check the Appropriate Box if a Member of a Group	(a) x (b) o
3)	SEC Use Only
4)	Citizenship or Place of Organization	Delaware
Number of Shares Beneficially Owned by Each Reporting Person With		5)	Sole Voting Power	91,781 shares of Common Stock
		6)	Shared Voting Power	-0-
		7)	Sole Dispositive Power	91,781 shares of Common Stock
		8)	Shared Dispositive Power	-0-
9)	Aggregate Amount Beneficially Owned by Each Reporting Person	91,781 shares of Common Stock
10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares
11)	Percent of Class Represented by Amount in Row (9)	0.2%
12)	Type of Reporting Person	PN

CUSIP No. 928645 10 0

1)	Name of Reporting Person I.R.S. Identification No. of Above Person (Entities Only)	Domain Partners IV, L.P.
2)	Check the Appropriate Box if a Member of a Group	(a) x (b) o
3)	SEC Use Only
4)	Citizenship or Place of Organization	Delaware
Number of Shares Beneficially Owned by Each Reporting Person With		5)	Sole Voting Power	2,743,370 shares of Common Stock
		6)	Shared Voting Power	-0-
		7)	Sole Dispositive Power	2,743,370 shares of Common Stock
		8)	Shared Dispositive Power	-0-
9)	Aggregate Amount Beneficially Owned by Each Reporting Person	2,743,370 shares of Common Stock
10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares
11)	Percent of Class Represented by Amount in Row (9)	5.8%
12)	Type of Reporting Person	PN

CUSIP No. 928645 10 0

1)	Name of Reporting Person I.R.S. Identification No. of Above Person (Entities Only)	DP IV Associates, L.P.
2)	Check the Appropriate Box if a Member of a Group	(a) x (b) o
3)	SEC Use Only
4)	Citizenship or Place of Organization	Delaware
Number of Shares Beneficially Owned by Each Reporting Person With		5)	Sole Voting Power	21,703 shares of Common Stock
		6)	Shared Voting Power	-0-
		7)	Sole Dispositive Power	21,703 shares of Common Stock
		8)	Shared Dispositive Power	-0-
9)	Aggregate Amount Beneficially Owned by Each Reporting Person	21,703 shares of Common Stock
10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares
11)	Percent of Class Represented by Amount in Row (9)	less than 0.1%
12)	Type of Reporting Person	PN

CUSIP No. 928645 10 0

1)	Name of Reporting Person I.R.S. Identification No. of Above Person (Entities Only)	Domain Associates, L.L.C.
2)	Check the Appropriate Box if a Member of a Group	(a) x (b) o
3)	SEC Use Only
4)	Citizenship or Place of Organization	Delaware
Number of Shares Beneficially Owned by Each Reporting Person With		5)	Sole Voting Power	55,238 shares of Common Stock
		6)	Shared Voting Power	-0-
		7)	Sole Dispositive Power	55,238 shares of Common Stock
		8)	Shared Dispositive Power	-0-
9)	Aggregate Amount Beneficially Owned by Each Reporting Person	55,238 shares of Common Stock
10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares
11)	Percent of Class Represented by Amount in Row (9)	0.1%
12)	Type of Reporting Person	OO

CUSIP No. 928645 10 0

Amendment No. 1 to Schedule 13G

Reference is hereby made to the statement on Schedule 13G filed with the Securities and Exchange Commission on February 12, 2007 (the “Schedule 13G”). Terms defined in the Schedule 13G are used herein as so defined.

The following Items of the Schedule 13G are hereby amended and restated as follows:

Item 2(a)

–

Name of Person Filing:

This statement is being filed by Domain Partners V, L.P., a Delaware limited partnership (“Domain V”), DP V Associates, L.P., a Delaware limited partnership (“DP V A”), Domain Partners IV, L.P., a Delaware limited partnership (“Domain IV”), DP IV Associates, L.P., a Delaware limited partnership (“DP IV A”), and Domain Associates, L.L.C., a Delaware limited liability company ("DA") (collectively, the "Reporting Persons").

Item 2(c)

–

Place of Organization:

Domain V: Delaware

DP V A: Delaware

Domain IV: Delaware

DP IV A: Delaware

DA: Delaware

Item 4

–

Ownership.

(a)

Amount Beneficially Owned:

Domain V: 3,885,309 shares of Common Stock

DP V A: 91,781 shares of Common Stock

Domain IV: 2,743,370 shares of Common Stock

DP IV A: 21,703 shares of Common Stock

DA: 55,238 shares of Common Stock

(b)

Percent of Class:

Domain V: 8.3%

DP V A: 0.2%

Domain IV: 5.8%

DP IV A: less than 0.1%

DA: 0.1%

CUSIP No. 928645 10 0

(c)

Number of shares as to which such person has:

(i)

sole power to vote or to direct the vote:

Domain V: 3,885,309 shares of Common Stock

DP V A: 91,781 shares of Common Stock

Domain IV: 2,743,370 shares of Common Stock

DP IV A: 21,703 shares of Common Stock

DA: 55,238 shares of Common Stock

(ii)

shared power to vote or to direct the vote: -0-

(iii)

sole power to dispose or to direct the disposition of:

Domain V: 3,885,309 shares of Common Stock

DP V A: 91,781 shares of Common Stock

Domain IV: 2,743,370 shares of Common Stock

DP IV A: 21,703 shares of Common Stock

DA: 55,238 shares of Common Stock

(iv)

shared power to dispose or to direct the disposition of: -0-

Item 8 –

Identification and Classification of Members of the Group:

See Exhibit 2.

CUSIP No. 928645 10 0

Signature:

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

	DOMAIN PARTNERS V, L.P.
	By:	One Palmer Square Associates V, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DP V ASSOCIATES, L.P.
	By:	One Palmer Square Associates V, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DOMAIN PARTNERS IV, L.P.
	By:	One Palmer Square Associates IV, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DP IV ASSOCIATES, L.P.
	By:	One Palmer Square Associates IV, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DOMAIN ASSOCIATES, L.L.C.

	By:	/s/ Kathleen K. Schoemaker
		Managing Member

CUSIP No. 928645 10 0

Date: January 23, 2008

CUSIP No. 928645 10 0

AGREEMENT OF

DOMAIN PARTNERS V, L.P.,

DP V ASSOCIATES, L.P.,

DOMAIN PARTNERS IV, L.P.

DP IV ASSOCIATES, L.P.

AND DOMAIN ASSOCIATES, L.L.C.

PURSUANT TO RULE 13d-1(f)

The undersigned hereby agree that the Information Statement on Schedule 13G to which this Agreement is annexed as Exhibit 1 is filed on behalf of each of them in accordance with provisions of 13d-1(f) under the Securities Exchange Act of 1934, as amended.

	DOMAIN PARTNERS V, L.P.
	By:	One Palmer Square Associates V, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DP V ASSOCIATES, L.P.
	By:	One Palmer Square Associates V, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DOMAIN PARTNERS IV, L.P.
	By:	One Palmer Square Associates IV, L.L.C., General Partner
	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DP IV ASSOCIATES, L.P.
	By:	One Palmer Square Associates IV, L.L.C., General Partner

CUSIP No. 928645 10 0

	By:	/s/ Kathleen K. Schoemaker
		Managing Member
	DOMAIN ASSOCIATES, L.L.C.

	By:	/s/ Kathleen K. Schoemaker
		Managing Member

Date: January 23, 2008

CUSIP No. 928645 10 0

EXHIBIT 2

Identification and Classification

of Members of the Group

Domain Partners V, L.P., DP V Associates, L.P., Domain Partners IV, L.P., DP IV Associates, L.P. and Domain Associates, L.L.C. are filing this statement on Schedule 13G as a group.

Domain Partners V, L.P. is a Delaware limited partnership. Its sole general partner is One Palmer Square Associates V, L.L.C., a Delaware limited liability company.

DP V Associates, L.P. is a Delaware limited partnership. Its sole general partner is One Palmer Square Associates V, L.L.C., a Delaware limited liability company.

Domain Partners IV, L.P. is a Delaware limited partnership. Its sole general partner is One Palmer Square Associates IV, L.L.C., a Delaware limited liability company.

DP IV Associates, L.P. is a Delaware limited partnership. Its sole general partner is One Palmer Square Associates IV, L.L.C., a Delaware limited liability company.

Domain Associates, L.L.C. is a Delaware limited liability company.