doubleofive said:
How does one get permission to use Vader's Personal Elevator? How could Vader be caught unawares if his wall dings that someone is headed down to see him? Just playing devil's advocate here (and I really like the hallway idea).

That one is easy: it's like the "basement" button in the Arkham Asylum elevator in BATMAN BEGINS. Only someone with the password or the key can unlock the button and use it, whereupon when someone uses it an automatic signal is sent to Vader in his chamber (so he knows someone is coming).

Also in regards to Vader's chamber, I would personally choose the elevator with airlock. Vader doesn't have to be 2 feet away from the bridge, being that with an elevator or hallway he could still be anywhere from 1 to 10 levels below the bridge and still within the command tower. I think the "privacy" factor is a more pertinent reason to change the background behind Piett than anything else.

Also also remember that when Luke took off the helmet in JEDI, the front face mask was still in place, which means Vader would still be using his mechanical breathing apparatus to... well, breath.

EDIT - it looks like the front IS attached to the rest of the helmet, as you can kind of see here (you can see it better when watching the movie):