Make things simple and go to Vanguard directly to setup the account. Get the mutual fund version. If you get the ETF you will have to pay a brokerage a commission every time you get new money that you want to add into the investment. Even if that commission is only $5 a trade, if you are adding money from your paycheck every 2 weeks to buy more shares, that is a bunch of money being wasted. In contrast, going through vanguard directly with the mutual fund (VTSAX) allows you to buy additional shares for free.
Also, there is really no point in trying to get a vanguard fund through USAA, it may or may not work. Bottom line is that it would be an extra hassle, will probably cost you more, and lead to plenty of confusion and stress, and an extra layer of management.
I know change can be tough, I used to have a brokerage account I used for years for buying and selling until learning the ways of MMM, I parted ways with my old technique of investing and setup an account with vanguard directly, it seems strange at first, but after a month or two it will become familiar and second nature to you.