* Names others to manage most of $3.7 bln U.S. Growth fund
* "What took Vanguard so long?" - Vanguard analyst
BOSTON Oct 7 (Reuters) - Vanguard Group Inc said on Thursday it has replaced AllianceBernstein Holding LP (AB.N) as manager of most of its $3.7 billion Vanguard U.S. Growth Fund (VWUSX.O) after a long stretch of subpar performance.
Vanguard, the privately held Pennsylvania fund giant, said it named Wellington Management Co and Delaware Investments to replace AllianceBernstein, which had run about two-thirds of U.S. Growth and had served as an adviser since 2001.
The remaining one-third of the fund will continue to be run by William Blair & Co, Vanguard said. "We are confident that the new advisors...will help the U.S. Growth Fund regain some of its past luster," said Vanguard Chief Executive Bill McNabb in a statement.
U.S. Growth's record has been a sore spot for Vanguard for some time. Though at times a standout, its returns rank it just average or worse over three-year and five-year periods according to Morningstar Inc.
"One has to ask what took Vanguard so long?" Daniel Wiener, who writes a newsletter for Vanguard investors, said in a note.
The fund's largest holdings as of June 30 included computer makers Apple Inc (AAPL.O) and Hewlett-Packard Co (HPQ.N), and search firm Google Inc (GOOG.O), according to Morningstar.
An AllianceBernstein spokesman declined to comment on whether the move would affect the company's earnings. The company had $467 billion under management as of August. It continues to help run other money for Vanguard.
"We remain deeply committed to our valued relationship with Vanguard," AllianceBernstein said in a statement. "We continue to provide several investment services to the firm and will work to further enhance our partnership with them."
Shares of AllianceBernstein were roughly unchanged in morning trading. (Reporting by Ross Kerber, editing by Dave Zimmerman)