Hi @ennoh,
It appears that the vent was just screwed into the plasterboard, which is not a good way to secure something and would explain why it has dropped. Usually, you would use a plasterboard anchor, but unfortunately, due to the proximity to the edge of the plasterboard, this would not work.
Is there a duct or anything above this vent that would stop you from attaching a piece of timber to the back of the plasterboard?
If not, then I'd suggest unscrewing the vent and removing it, grabbing a piece of timber such as this 70 x 35mm 0.9m Short Pine Run Of Mill, placing it on the top of the plasterboard so that the outside edge is in line with the outside edge of the plasterboard, then drive some 30mm Plasterboard Screws through the plasterboard into the timber. Make sure the plasterboard screws are about 25mm back from the edge to prevent the plasterboard from cracking. This will lock the timber in place.
Once the timber is in place, you can screw through the plasterboard into the timber. It looks likely that the existing screws are too short to punch through the plasterboard, so you may need to use longer ones. The plasterboard screws you used for the timber would be fine, or you could use 30mm White Button Head Screws if you'd like them to be less noticeable.
If there is anything that would prevent you from installing a timber, could I trouble you for a photo of the cavity, as this will help me work out what you could do?
Let me know what you think.
Jacob
