I have velvac mirrors and have had the same problem. I went to the velvac.com and downloaded the exploded view drawings for the mirror. On my model of mirror, I removed a cover plate and there was a hex screw, had to turn the mirror in the opposite direction to be able to tighten the screw, then, pulled the mirror back to the screw threads.....I know it sounds confusing, but it will make sense after you see the inside of your mirror.
2012 Tiffin Phaeton 36 GH
2013 GMC Sierra SLT 4x4 (flat towed)
Honda Pioneer 500 (UTV)