I make boat decals and designs, the vinyl doesn't come off the boats in 99.9999999% of cases, but if the person who is applying them has not prepared the area before trying to fix them, they may not stick as well. Causes of this are usually grease off the skin, releasing agent as used in moulds for plastic hulls or simply a rough surface.
As for varnishing, the decals do look better if they are sealed into the 'skin' of the boat and will have a smoother edge to them. Personally, I use Acrylic Lacquer spray from Halfords for a gloss finish, but I also have matt lacquers as well.
Nige