My wife has a cutting machine for crafting, and she cut out vinyl letters for me a couple years ago, they are still going strong.
I cleaned the hull really well, sprayed a base coat of Krylon Plastic clear coat onto the plastic hull, then place the letters over the dry clear coat, then I sprayed a heavy coat of clear coat over the letters to seal the edges well. I think it would be difficult to get the letters off now.
After putting on 2 sets of vinyl registration numbers without the clear coat, and having them just fall off within a few months. I did a similar clear coat with my vinyl registration numbers on the hull, they are also still going strong as well.
On my first two TI's I had the registration numbers spray painted on ( I used stencils), because I didn't think the vinyl numbers would stay on. What a pain that was and it didn't hold up at all.
The name of my boat is the Ultimate Tandem Island (the word Island is replaced with a pic of an Island)
Hope this helps
Bob