Got the Vex Mythoclast drop last night! I've been pretty lucky so far w the drops, this was my 6th attempt at Atheon on hard. I did notice something that might be just a hunch regarding the droprate for it. I think you're better off joining a group with a checkpoint before Atheon. My guess is each run is only allowed so many Vex drops, for example if a member who started the raid finishes Atheon with other new members then that "raid" already had its drop. So joining other members who also have the checkpoint would no longer see any Vex drops when they clear Atheon. Again, this is just what I noticed with my few runs but if you've been at it for a while give it a try!