Hi, Phil. A part of the answer depends on the look you want. If you want to hide them, it is OK to use the wall paint on the top but remember that using a light color will let dust collect easier and show more. I would start with a spray coat of rust preventive primer that was definately compatable with the paint you planned to use. Watch out for Rustoleum since their fish oil base can cause compatability problems. Check an automotive supply store for rust preventive paints that might be more compatable. They will also have a variety of colors including metallics that might work better for you.
One thing you might want to note for the future is that the vents are real cheap at a home center. Unless you have something really special, it might work out better just to replace them rather than strip them.