You seem to only have a few VOG completions and I have to assume some are on easy mode. Check my profile I have an actual tonne of completions and three vex to show for it. If you do VOG enough you will get a vex. it doesn't only hand out one per completion it can be none, one or many. Percentage does seem to have gone down recently though as I've noticed so perhaps it's been patched into oblivion given that (come 19th) it's going to be a seriously lethal PvP weapon again.