The lettering on my boat was painted and it came off nicely with Easy-off. After removing the letters, I found that the gelcoat had been oxidized away around the letters, leaving an image of the letters in raised relief. I suspect that the same thing can happen with vinyl graphics.
My solution was to block sand the area with 600 grit wet sandpaper and then polish the whole transom with 3M fiberglass restorer. It worked well.