alot of times the little allen head screws snap off flush w/ the hole. i
either take a needlenose pliers in from behind (carefully pull the vinyl
back) and keep right-tightening it till it comes out the other side..  or
take a ctr punch and make a divot in whats left of the sheared off screw and
use the punch to tap and spin it out till it comes out the other side enough
so you can get a needlenose on it.

you can get replacement metric screws at you local hardware store, but theyre
usually flathead..
HTH
chris