Based on my experience, this honestly sounds like a VA policy. I'm on Bupropion through the VA (for possibly similar reasons as you'd want Vyvanse) and they are particularly stingy about it, only giving me a 30 day supply at a time.
Even with a different provider at the VA, you may get the same result.
Just make sure it's RFC 2324 compliant. You don't want it throwing any HTTP 418 error messages.
If you're having power on issues, I would make sure you are using a standalone power brick. I've used a Surface Pro for work and I've found that sometimes when connected to the docking station, it wouldn't power on. When connected to a standalone power brick it usually would. For a standalone power brick, I've used both the one that came with the Surface and also a 65W USB-C adapter with a USB-C to Surface per cable.
This honestly sounds like the best solution to this particular problem.