I fitted a Vamoose rail a couple of weeks ago. This comes with self drilling screws which you use to make the holes and bend the rail to the contour of the van whilst these are fitted. You then remove these screws and rail and Sikaflex and refit with different normal type self tapping type screws. When I refitted the rail a couple of the screws spun like you have described and I was worried about over tightening the others and the same thing happening. I therefore think that the Sikaflex bond is giving the majority of the fixing strength.
