If the ceiling feels spongy anywhere at all, some sort of deterioration has occurred. The most common cause is moisture seepage—whether from leaky plumbing, roofing, or pets—but plaster deterioration can also occur from sustained high temperature (120 °F/49 °C) as is common above an enclosed light fixture or electrical junction box carrying a heavy electrical load.
Decay could also be caused by certain pests like cockroaches, fleas, or larger critter's waste.
The first step to fixing it is to stop whatever is causing the problem. Then assess what needs to be done.
Wall and ceiling work is somewhat expensive if you hire someone to do it, but it is quite inexpensive if you do it yourself. Even if you don't know what you are doing and have to (re)do it four times, that will still be less expensive than hiring a pro. For a head start, visit your local Home Depot the next time they have a sheetrock demo. Or search for "sheetrock repair" on Youtube.
If investigation or repair require tearing down the ceiling, the price can escalate if you see something else you want to do: rewire, insulate, reroute plumbing, redo siding, etc. Frankly, the add-ons are the most difficult (financially and emotionally) aspects of such projects.
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.
Best Answer
Half a centimeter? In that case, spackle, sand, and paint. They make color changing spackle (pink to white) so you know when it's dry. Apply with a flat edge (e.g. a putty knife). Don't oversand, just enough to knock any rough edges off. And since the paint has an orange peel look, use a small roller for the touch-up.