If you're thinking of painting over the vents, please don't. It becomes a pain for whoever is going to paint the room the next time. It's not difficult to remove the vents before painting; it's certainly easier than removing them after they've been painted over. It's even easier to put masking tape around the edges, though you can't then be as cavalier with the roller. Actually, it's easier to remove the vents — two screws, pull, and you're done.
If you really want the vent covers to be the same color as the wall, remove them first, paint them separately, let them (and the walls) dry, and then replace them. It's more work, sure, but you won't then have painted the damned things to the wall. The better option is still not to paint them, so the next person doesn't have to buy replacements.
This has been your grumpy homeowner's rant for the day.