The key thing here is to make sure USAA clearly understands that you do not wish to settle and that you intend to fight the total loss. Insurance companies metric how quickly their agents close claims (because they know the longer it draws out the more likely it is they will face legal action) so you'll probably find them more pliable the longer you hold out. The problem is there's a law in place that's designed to prevent you from essentially blackmailing them into doing what you want; it's called "loss reduction." My rough understanding is that if you delay closing of the claim for an unreasonable length of time such that you incur significant additional costs for the insurance company (in your case by keeping a rental) they can force you to take their settlement, or deduct some portion of their costs from their original settlement offer. Of course you can challenge that settlement but in my research I found that the typical person can't afford those legal bills.
Do you have the ability to get to work without a rental car? If so you might consider dropping the rental car to gain leverage to keep your VX from getting totaled. If your rental is costing USAA $50/day and all they need to do is raise the value of your VX by a few hundred bucks to keep it from being totaled that should be a no-brainer for them. I would ask for a supervisor the next time you talk with USAA and see if you can negotiate something. But all that's after they provide a reasonable answer to why they can total something that has a repair cost less than the 75% stipulated by Virginia law.