I’ve done glass frit and spar urethane on top of vinyl before. That’s how this one was done:
https://www.instagram.com/p/B-SJqcBJqO8/
That was a full top side application, and I asked the customer to let me know how it wears. I have not heard back from them yet.
also i regularly die cut vinyl for top side applications with poly and frit on top of it and so far no issues there.