Interesting topic. When does the vault close? Does it stay open if nobody dies? Can one player stand in a bubble in the doorway to keep it open? Maybe after the atheon kill the portal outside the vault becomes active? Travel the portal and there is something to be found.. Who knows? I would like to. I think the dev team should drop hints until it is found. Maybe this is why the fight originally teleported the three characters that were furthest back? They didn't want anyone holding the door open? Just my speculation.