While I had the vent covers off, I decided to do something about the yellowing of the covers. In broad daylight, it was very evident that these were quite yellowed.

At first, I used a Magic Eraser, which did an OK job and you could tell it was working. I decided to spray my favorite cleaner, L.A.'s Totally Awesome, on it. As soon as I sprayed it on, the discoloration practially fell off.

"Cleaned" is on the left, "before" is on the right.