Schwab is comparable with Vanguard in terms of fees and ETF availability, so I never bothered with opening up an extra account at Vanguard.
I use Wealthfront to diversify my ETFs. They take care of distributing the money I put in the account among several areas of the market (U.S. Stocks, Foreign Stocks, Emerging Markets, Dividend Stocks, Natural Resources, Municipal Bonds and Cash).
They mostly use Vanguard funds. I could probably also do this myself, but I don't really want to manually balance my portfolio every time asset classes go up and down. So far I only put the amount of money in there that they manage for free. If you intend on signing up, one word of caution: I signed up on their homepage and had 10k under free management. If you sign up via a referral link, you get 15k. I'm still slightly annoyed that I didn't use one when opening an account. I know that it's against the usual HN policy to post them, but since it is detrimental to sign up without one, here we go: http://wlth.fr/196dDW2
(I'd check with my friends first to see if one of them is already on the platform)
p.s. this is US centric.