Pay attention to Vader after he dispatches Palpatine; he is barely able to move, and needs Luke's help to get to the hanger bay. Easy to write off as injuries due to taking the brunt of Palpatine's lightning attack. Episode 3, however, reveals that all of Vader's limbs areare, at this point, artificial, and electrical...easy to short out due to an electrical overload by, say, sith lightning? Vader needs help to move because he literally can't at that point. He also knows that he'll die even if his mask stays on because, again, the life support system has been blown to hell.