You might just consider getting either a black or a white vent cover. You'll probably stop noticing it if it blends in better. If you do get a new cover that is metal, you can use any quality enamel. Two coats, allowed to dry thoroughly between coats, should resist chipping and wear for a long, long time. As the guys at your local paint store to be sure – they may recommend using a coat of metal primer first. (And vinyl or other plastics are much harder to paint.)
But if you do this, keep the old vent cover on until the new one has plenty of time to finish curing, preferably outdoors in a well-ventilated area. This can take awhile if the paint skins over quickly.
I had a door I painted that took over 3 months before I could no longer smell it. I used special "natural," low-VOC paint that was supposed to be low-odor, because I have pretty severe chemical sensitivities. I'm pretty confident that it's not toxic now, even when the sun is hitting it.