I'd like to think that it is also affected by your Grimoire score, not in the sense that the higher the better, but that your RNG changes with your score. To me, that explains why long time players experience the same drops over, and over (little to no score change). While, newer players seem to have all the luck (more chance at score change). I just got the vex twice in two weeks on my first and second HM completions. My Grimoire is just over 2k atm.