The chamber is primarily a hyperbaric chamber of sorts that allows him to free himself from his helmet. I have a few ESB production pictures on my HD that show Vader sitting in the chamber sans helmet, but there is a an apparatus into which he breathes. It is a face mask of sorts that protrudes from the mechanism that lowers his helmet onto his head. Clearly these shots of him with this "oxygen mask" attached to his face never made it to the final cut. However, the fact that he was breathing into this thing, which actually kind of looks like an in-air refueling basket for military planes, means that the front of his head would have had no mask and that what we see descend from above is his entire helmet, not just the dome.