What if.. and its a pretty big IF
It’s actually the Doctor who’s locked in the vault?
I’ve noticed over the course of Series 10 that we have had a slight change of personality from the Doctor, and he wasn’t able to cure his blindness in Oxygen.
What if its a fake Doctor ( lets just say created by the Autons, Daleks OR Missy herself..) that’s actually flying around with Nardole and Bill?
Could be plausible.