[TromboneAl]

Quoting from an old post:

I thought about this a few years ago, but decided the very, very small risk from having everything at Vanguard was outweighed by the advantages and convenience.

Here's what Vanguard has to say about this (full link here):

Diversifying by fund family: Limited benefits

Another misapplication of Sancho Panza's advice is investing with more than one fund company to protect against the risk that financial troubles in one fund will infect the fund family's other funds.

Such a scenario is impossible. Each mutual fund is its own corporation. A problem at one mutual fund wouldn't affect another, even if both were managed by the same company. If you own two funds from the same fund family, you have, in effect, put your eggs in two different corporate baskets. In addition, mutual funds are required by law to keep fund assets with a third-party custodian, large well-capitalized banks hired to safeguard shareholder assets. Daily checks are made between mutual funds and their custodians to make sure that assets are where they're supposed to be. The custodian banks, in turn, carry insurance coverage on their own to cover losses of securities and cash in their possession. And Vanguard, like most mutual fund companies, carries substantial insurance resources to protect against any illegal acts. (Thanks to Vanguard's vigorous fraud-protection efforts, though, we have never had to draw on this insurance.)

What about risks to your fund company's recordkeeping and administrative operations? Vanguard's approach has been to develop extensive contingency plans, backup systems, and multiple geographic locations. The Vanguard® funds' trustees are continually working with Vanguard's leadership to ensure that our operations are the industry's most secure.