I've provided "base info" to BF member JRrestore (aka VeloCals.com) who does custom reproduction, for a couple projects. He seems to be happy enough to have digital photos taken in situ from several angles (such as: multiple shots head on taken radially around a tube). No matter what you do, it's de rigeur in the conservation game to scrupulously document original state before proceeding with anything irreversible.

There are several sorts of graphics that vintage bikes might have: printed vinyl is fairly recent, while older bikes had graphics printed on plastic/foil composite (Gitane, Jeunet), and in true "decals" there were wet transfer, and dry transfer types either manufactured with their own adhesive, or to be affixed with shellac or some other binder. Whether one can get them off depends on the type, and whether they were clear-coated. Kommisar89 is quite correct in saying that you should expect some distortion, even if you can remove the graphic whole. That's why pre-mucking-around photo documentation is important.