I'd use a few pieces of wood up in the ceiling, to have something for screwing the vent to. The plaster is getting a little worn-out there. I wouldn't trust anchors or make anymore holes for them, nor use toggle bolts; that plaster is about to go and you're about to run out of room (hide-able hole). -I just really don't like wall hangers and I wouldn't want to be pounding some in at that location. It's also not going to be the easiest thing to line up all 3 holes' anchors.
You may get lucky and rotate it just right to find something to bite (don't rip the duct) or just get some shims up there and be done with it. If you do rotate it and find undisturbed plaster to screw to, use drywall screws and not the silver ones you probably have from the vent (larger thread; better grab -do not over tighten them).
Judging by the number of holes, this is the fourth time it's come down; shim it.