Ditto on the cleaning, but defroster vents should just be dusty/dirty unless they've been sprayed with Armorall or another detailing spray that contains silicone.
The defroster vents weren't painted suede, just molded in an approximate match of the dash color.
I've never had a problem with painting them, and there are door panels out there I painted 30 years ago and they're still mint.
I have the paint, left over from a project; I can paint them for you if you like, just mail them to me - no charge.