I've been a Fidelity customer (Gold level) for over 20 years, but nothing very mustachian about them. I still have accounts there for option trades, but do most of my mustachian investing at ShareBuilder.
I started a ShareBuilder account with the CostCo card member discount (see
http://content.sharebuilder.com/mgdcon/jump/partner/costco/pagec/?cobrand=costco&execpc=COEXWEB2012&gspc=COGOWEB2012&offer50=true&gssid=960036629097109&execsid=960036611000791&execmpid=0954&gscmpid=0955&sPageName=sb:mgdcon:Partner:Costco-lpa2009), and it saves on every trade.
Prices Standard CostCo
Basic Trade $6.95 $5.95
Adv. Trades $6.95 $4.95
AIP trades: $3.95 $2
Advantage: $12 $10
I signed up for the 'Advantage plan' - 12 AIP trades/month for $10 (per account), $2/trade after that. I do mostly "Automatic Investing" there for myself, and my sons' custodial accounts. ShareBuilder does NOT offer VTSAX, but offers VTI (in the same Vanguard fund family).
The best thing Sharebuilder does is 'automatic investment plans' - regular, set-it-and-forget-it investing for very low transaction fees - lower for CostCo members. ShareBuilder allows you to buy partial shares of companies (i.e. fixed dollar investments) on a regular basis - ex: buy $100 of GOOGL stock as if it were a mutual fund, and do it weekly, twice monthly, or weekly.
As for buying VTSAX... there's a $10,000 minimum first investment. Why not open an account at Vanguard strictly to invest in VTSAX?
[I have no idea what Vanguard's trading fees are like, but I'm sure they're equitable]