Why Vanguard? Because of vanguard's ownership structure. It's not stockholder owned like Goldman Sachs or Morgan Stanley, who are known to A: Sell bad products to their clients and do many other things to transfer wealth from client to broker or B: Trade stocks often in order to get more active management fees. It's not family owned like Fidelity, who does try to provide better services possibly because of that reason, but still wants to make profit.
Vanguard has a completely unique management structure. According to wikipedia: It's owned by the funds themselves. In a sense, getting an account with Vanguard means that Vanguard is answerable to you, not to people trying to get a piece of the pie. They extract only the money required from their funds to continue existing, and they're very good at it.
Basically, if you want to get index funds, the best are ranked kind've like this:
1. Vanguard
2. Blackrock/Fidelity/DFA
3.
4.
5.
6.
7.
-10
-20
-30
-Hell: G.S., M.S.
(D.F.A. is Dimensional Fund Advisors, they're only available through brokers, but are like a smarter take on Vanguard funds because they're not pigeonholed into following an Index, but the broker takes a cut to so it's harder to get into them. Blackrock owns iShares now which is a very good fund series, and from what I've seen of them, they work very hard not to be noticed and do provide better services for less costs on their funds).