on really old crusty stuff, use a razor blade to remove the vinyl - leave the crusty adhesive underneath. Then take a rag soaked with acetone and lay it on top of the adhesive for a minute or two (the rag keeps the acetone from evaporating so quickly). Then, carefully, use the razorblade to scrape up the adhesive - it will come up like butter. Keep a clean rag to clear the razor blade often. Then you can rub down with acetone to remove the little tiny leftovers.