I locate the new hole by inserting a small screwdriver or awl from the inside, through the bracket's threaded hole, carefully maintaining the correct angle. With a little push I can mark the location with an ink dot or simply push an awl through the vinyl. I then take a heated soldering iron and burn a hole about the size of the anchor's threads — this also burns the edges of the new hole to prevent future fraying. A 6mm allen will seat the anchor bolt. Quick and easy.
(click on an image to enlarge)