You need to talk to a VA Rep - there's an awful lot of information you're going to need answers to. I'm retired military, 50% disabled and I have Tricare for Life and Medicare. Because I'm retired, I don't need Part D because I get my prescriptions through Tricare (although I get one through VA because Express Scripts didn't carry it when I started on it). Since I'm on Medicare, it's considered my primary insurance and Tricare is secondary.
Most of the time, I go to our VA clinic - mostly for semi annual checks and prescription updates, and shots. Medicare pays first, Tricare second, if there is a second - no co pays at all. My prescriptions run $7 per script for a 90 day supply.
I also have another doctor (a friend of ours) and while I've only been to her office four times, I've not had any co pays.
When I started out with VA they assigned appointment priorities by range of disability and type of service. Being retired and with the 50% service connected disability, I fall into priority 1. As I understand it, a tinnitus disability usually only results in a 10% or 20% disability, so if that is the case, you would be a low priority when you requested an appointment. Your ability to get prescriptions might also be affected by the degree of your disability.
Since you don't appear to be retired military, you probably will fall into a low priority group and you may or may not be able to obtain prescriptions through them and your income status may affect your co pay status.
Do not drop your Part D until you get all your questions answered to your satisfaction. And do the Ronald Reagan thing after talking to whichever VA Rep you talk to - trust, but verify. I've seen a couple that can't answer specific questions but simply hand you a bunch of pamphlets and tell you that you'll find the answers in them.
Go to the VA website, check out the FAQs - but be darned sure you have correct answers before you make any moves. One wrong one could cost you a bunch of money