yes, I was going to ask how you painted the vinyl as I know its not that easy. Door trims get kicked, rubbed and scratched all the time, and unless its painted professionally, than I would say all you are doing, is creating a nightmare for yourself.
Questions:
Did you primer before you painted the surface? (Plastic Primer?)
Did you use the right paint for the right job?
Did you apply a few coats of paint, or just one?
Did you allow the coats to dry before reapplying?
Did you spray clear coats at all to help prevent visible scratches?
Thats all I could think of.......
PS: Its not a bad idea to ask a panel beater to spray your trims for you as they have the right gear, and SHOULD follow the right process.