Shooting clear over vinyl works just fine and lasts indefinitely, if you're careful and know what you're doing. The biggest trick, and the short explanation, is to spray the initial coats "dry" and light with drying time allowed between those coats. Do so until you get things sealed and covered enough to get yourself into a safe zone for a heavier final coat. I've done this on countless models with consistent success. My "YIPPEE" P-38 is just one example, other than the canopy detail all the other graphics are vinyl sealed and finished with Brodak clear.