Except this clearly isn't the case. Vek'lor doesn't move. Vanish appears to only be used for HP recovery due to dealing with the two adds that have been brought in - and this has me suspicious. These guys hit reasonably hard and there would be no normal reason to drag them into the fight as well.
edit - the defender has a port player ability, I wonder if he pulled the bosses first before starting the recording, got ported back to the defender, and they reset in some state?