77 is right, just clean it and let it dry, then spray it. Only thing I would add is to use some vinyl prep (adhesion promoter) before you spray the color on it.
Like he said, if you have different paint on it then your new vinyl paint may not stick. You can either risk it and go ahead with the cleaning, spraying or you can try to strip it down. The stripping might be rough on your plastic, not sure. Or you can do what you were saying and rough the old paint up with some 'paper, then do the other steps.