factory door panels aren't actually vinyl...they are unsupported plastic film...i.e. they have no backing fabric to give them strength...you can heat out small wrinkles but apply too much heat and you will stretch the cover or burn it...it really depends on how bad they are in the first place...I've unglued the covers around the edges and heated then stretched the cover back around the boards to tighten the covers a bit, again not too much heat nor stretching...if you have an old panel have a play with that first!!
Soak in a bathtub full of hot water. When they get soft, lay them out on a flat surface, cover with a towel and then use books or other weighted objects to hold them flat.
Let sit for a day, and they come out looking like new