ceiling vent drywall crumbled nail out best way to repair

@deboraq sorry for the several-month delay.  I would try to get a screw 1 or 2 sizes larger to see if I could get the new (larger) screw to hold of the metal flange of the duct work.  I am assuming there is a flange that the old screw threaded into and that is now stripped out making the standard screw loose.  

Did something like that and angled. It worked. Now just have to paint the heads white.

Thanks

Use a drywall anchor or toggle bolt to secure the vent cover back in place without needing to replace the drywall.