So from my experience players usually get it between 10-30 runs and players that run it more without it are players that have unlootable clears. I dont know if its bugged but dont count it out considering someone claims 400+ clears which is pretty insane.

If youve done it ~20 times between 6 of you and it still hasnt dropped id personally consider it bugged. Vex is supposed to be a 5% drop rate increasing with every lootable clear which would likely put you a good bit above 70% chance per person to get it and theres potentially multiple of you. Anything is possible but there comes a point where its so improbable its unrealistic to keep chasing unless you want to believe otherwise and it does work.