Drywall before or after vent installed?
Put up drywall first - it is easier to get it parallel to close walls since you could install drywall 1-2% off.   If you are not attaching this to a run and it is an open return I would also suggest trying to get at least one side next to a stud.   Often the support holes for these are close to the opening so even with a good anchor these get flimsy.