Just keep trying. RNG can't be helped so you just have to keep testing it. As for me, I would like that and a Mythoclast. I only have about 5 hard clears, but never got the chest until it was too late, so I know your pain.
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ah man, I got vex after like only 3 or 4 hard runs. I think I have had 3 total over my 24 runs. I like vex for PVP, not so much for PVE unless it's a solar burn night fall.