Board Thread:Lore Discussion/@comment-4057475-20130710075111/@comment-25047024-20131230114840

I think masks are more like a gift from the dragons which shows their rank in the Dragon Cult. As far as we know all the Dragon Priests with masks in Skyrim were High Priests. There are other, 'normal' priests we see in Skyrim who don't have a mask. So I guess that Vahlok was one of the normal priests who still fought against Miraak.