Just go to
www.vanguard.com and open an account directly with them. Another option is to open an account at a discount brokerage such as ameritrade or scottrade and buy the vanguard fund through them. If you are going to exclusively buy vanguard funds in your roth account, going through vanguard is best, but if you might want to buy stocks or funds from another family of mutual funds, go through one of the discount brokerages.
-xtravistx