I would stick with USAA if they have lower and better options.
I disagree. USAA is hard to beat as a bank but when it comes to mutual funds offered, I found that their expense ratios were higher than those at Vanguard. I'd consolidate the Roth IRAs under Vanguard.
To the OP: why do you have stocks under Vanguard and not mutual funds? And your diversity is determined by your overall asset allocation. Take a look at your total balance between your 457b and Roth IRAs. Determine how much of your total portfolio is in stocks (including stock mutual funds), bonds and cash. When you're done, combine the IRAs, dump the stocks and buy indexed mutual funds that will bring your total asset allocation to what's appropriate for your age and risk tolerance.