Pivot Bar Screws are Stripped

You might want to purchase new pivot bars or take a screw and pivot bar to your nearest hardware store. Tell them the problem that the screws are stripped. They might move you up to the next size thread screw. You might also go with a truss head screw instead of flat head screw.

I would first try using the larger screw before drilling the vinyl. Vinyl has a lot of give, so the larger screw might make its own thread through the vinyl then to the pivot bar.