Form 485BXT VANGUARD INDEX FUNDS

Explanatory Note

This Post-Effective Amendment No. 159 under the Securities Act of 1933, as amended (the “Securities Act”) (Amendment No. 160 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 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 September 1, 2017, the pending effective date of the Trust’s 157^th Post-Effective Amendment. By way of further background:

• Post-Effective Amendment No. 157 was filed under Rule 485(b)(1)(iii) on July 5, 2017, for the sole purpose of extending to August 4, 2017, the then-pending effective date of Post-Effective Amendment No. 156.

• Post-Effective Amendment No. 156 was filed under Rule 485(b)(1)(iii) on June 6, 2017, for the sole purpose of extending to July 6, 2017, the then-pending effective date of Post-Effective Amendment No. 154.

• Post-Effective Amendment No. 154 was filed under Rule 485(b)(1)(iii) on May 8, 2017, for the sole purpose of extending to June 7, 2017, the then-pending effective date of Post-Effective Amendment No. 152.

• Post-Effective Amendment No. 153 was filed under Rule 485(b) on April 26, 2017, to provide the annual update to the Trust’s Registration Statement.

• Post-Effective Amendment No. 152 was filed under Rule 485(b)(1)(iii) on April 10, 2017, for the sole purpose of extending to May 10, 2017, the then-pending effective date of Post-Effective Amendment No. 151.

• Post-Effective Amendment No. 151 was filed under Rule 485(b)(1)(iii) on March 10, 2017, for the sole purpose of extending to April 11, 2017, the then-pending effective date of Post- Effective Amendment No. 150.

• Post-Effective Amendment 150 was filed under Rule 485(a)(1) on January 11, 2017, for the purpose of adding the expense ratios for the Institutional Shares and Institutional Plus Shares of Vanguard 500 Index Fund. Post-Effective Amendment 150 originally requested that the Amendment become effective on March 13, 2017.

• Post-Effective Amendment No. 149 was filed under Rule 485(a)(1) on December 23, 2016, for the purposes of (1) adding Institutional Shares and Institutional Plus Shares of Vanguard 500 Index Fund, a series of the Trust, and (2) effecting a number of non-material editorial changes.

This Post-Effective Amendment No. 159 incorporates by reference the information contained in Parts A, B, and C of Post-Effective Amendment No. 157 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 2nd day of August, 2017.

VANGUARD INDEX FUNDS
BY:___________/s/ F. William McNabb III*_________

F. William McNabb III
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

Anne E. Robinson, pursuant to a Power of Attorney filed on October 4, 2016, see File Number 33-32548, Incorporated by Reference.