If you have a mirrored wall with an air vent cover, you really should have an Acrylic Mirrored Vent Cover to make the wall spectacular in appearance. Arnev Products offers a wide variety of sizes to fit your need.
The easiest way to install an Acrylic mirrored vent cover over your glass mirror is to use thin double-sided tape. The trick with this is to put the tape on the backside of the vent cover at least 1/8 inch away from the edge of all sides on the vent cover. The reason for this is if you ever have to take the vent cover off to clean the inside of the vent, by pulling off the cover you could peel off the mirror material on the backside and ruin the cover.
You also could use Velcro if you need to remove the cover or switch plate frequently for cleaning, but you may get a floating effect with the cover, and it will stick a bit farther away from the mirror with this technique, but it should work just fine.
These acrylic mirrored vent covers are available in many heights and widths and you could see all of our sizes on the acrylic mirrored vent covers page, so check them out.