Which models can this be used with?
Aria LiteDrywall Lite Bead, Drywall Lite Frame.

Can I mount this after I install my vent?
It depends on the vent. For the Aria Lite and Drywall Lite Frame, you can remove the vent from the duct, and clip on at any time. For the Drywall Lite Bead, the damper must be clipped on before installing the vent into the wall. 

Can I remove this once I've mounted it to my vent?
No. Once this is clipped onto the vent, the damper cannot be removed.