Ive tried painting some boats seats (vinyl) with the duplicolor vinyl paint and it doesnt last!! It will work on solid surfaces (like the door panels) but its a disaster on anything that flexes (like seats). You will have cracking and peeling and it will get on peoples clothes (which is embarrasing) plus it will look like crap. I hate to rain on your parade but seats will need to be replaced if you want everything to look good.
2006 M3 Convertible with SMG