If rng was perfectly random then you would have a 1 in 6 chance of getting a vex. Every hard completion I've had, at least 1 person got it (I think 2 people got it on one of my completions). Yet in my 25 completions, I've only gotten it once and over half the time it goes to the person who is dead and/or the lowest level. I got mine on my second attempt and I was a dead 29.