Why not Acetone?
I removed the old lettering on my boat last spring and tried goo-gone. It worked, but it was a lot of work. Acetone works much better, and I understand that it is much harsher on the finish, but as long as you rewax the portions of the hull not covered by the vinyl, I would think it is the most time effective/efficient. I may be naive about the potential damage, but as long as you use it in a sensible way, it seems to be the most efficient.Chris D