Before you resort to solvents, try heat on the decals. A hairdryer set to high and played over the decals will often soften the adhesive as well as the vinyl and let you peel them off in one clean pull. Then you might only have a little residue to clean off with a solvent, and Cirtasolv, Goo-Gone, or WD40 are usually good for this, really stubborn stuff can take lighter fluid (basically Naptha), but none of these *should* cause any harm to cured paint.