Clearcoat over actual decals or the vinyl stickers people today think of as decals will work just fine. At Evan's place I just saw an example of this which was done very well. Painting pieces of vinyl and then removing them from the release paper and trying to apply them without the paint cracking and coming off in the process is another matter entirely.
If you have a large vinyl sticker to apply (say to a hood or a door), try doing it wet. The water will keep the adhesive from sticking and will allow you to position it more easily. Once the water dries, you can gently press down and it will be stuck.
