Form 485BXT VANGUARD INSTITUTIONAL

Explanatory Note 

This Post-Effective Amendment No. 81 under the Securities Act of 1933, as amended (the “Securities Act”) (Amendment No. 82 under the Investment Company Act of 1940, as amended (the “1940 Act”)) to the registration statement on Form N-1A (the “Registration Statement”) of Vanguard Institutional Index Funds (the “Trust”) is being filed pursuant to paragraph (b)(1)(iii) of Rule 485 under the Securities Act solely for the purpose of delaying, until November 17, 2023, the pending effective date of the Trust’s 80^th Post-Effective Amendment.  

By way of further background, Post-Effective Amendment No. 80 was filed under Rule 485(a)(2) on July 21, 2023, to register shares of Vanguard Extended Market Index Fund, a new series of the Trust (the “Acquiring Fund”), in connection with a reorganization of a predecessor fund (Vanguard Extended Market Index Fund, a series of Vanguard Index Funds) with and into the Acquiring Fund. The Acquiring Fund is identical in management to the predecessor fund, and the investment objective, strategies, and policies remain unchanged. 

This Post-Effective Amendment No. 81 incorporates by reference the information contained in Parts A, B, and C of Post-Effective Amendment No. 80 to the Registrant’s Registration Statement. 

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant hereby certifies that it meets all requirements for effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the Town of Valley Forge and the Commonwealth of Pennsylvania, on the 19th day of October, 2023.

VANGUARD INSTITUTIONAL INDEX FUNDS

BY: /s/ Mortimer J. Buckley*

Mortimer J. Buckley

Chairman and Chief Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to the Registration Statement has been signed below by the following persons in the capacities and on the date indicated:

Signature	Title	Date
/s/ Mortimer J. Buckley*	Chairman and Chief Executive Officer	October 19, 2023
Mortimer J. Buckley
/s/ Tara Bunch*	Trustee	October 19, 2023
Tara Bunch
/s/ Emerson U. Fullwood*	Trustee	October 19, 2023
Emerson U. Fullwood
/s/ F. Joseph Loughrey*	Trustee	October 19, 2023
F. Joseph Loughrey
/s/ Mark Loughridge*	Trustee	October 19, 2023
Mark Loughridge
/s/ Scott C. Malpass*	Trustee	October 19, 2023
Scott C. Malpass
/s/ Deanna Mulligan*	Trustee	October 19, 2023
Deanna Mulligan
/s/ André F. Perold*	Trustee	October 19, 2023
André F. Perold
/s/ Sarah Bloom Raskin*	Trustee	October 19, 2023
Sarah Bloom Raskin
/s/ Grant Reid*	Trustee	October 19, 2023
Grant Reid
/s/ David Thomas*	Trustee	October 19, 2023
David Thomas
/s/ Peter F. Volanakis*	Trustee	October 19, 2023
Peter F. Volanakis

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