Vinyl is easiest to remove with a razor blade, and given the two options, I'd prefer to remove it from the hull in that fashion and not the sail
A hair drier or heat gun (if your careful) works well for removing. Softens the adhesive and the vinyl making it easier to peel off. If it's really bad take a wooden paint mixing stick. 45 degree the end and use it to scrape while applying heat.
Obviously this is not for sails.
