Stories featuring a suitless Vader seem to be a trend in fanfiction; I have noticed several running concurrently, two here on LJ and one elsewhere. Quality of writing notwithstanding, I have a great deal of difficulty with these stories because it is the suit that makes Vader. While it is indisputably more visually enjoyable to imagine Vader in his whole and beautiful pre-Mustafar state, to negate the serious and crippling nature of his injuries by making the suit merely a disguise, or by eliminating the suit altogether takes away the very core of the character.
Certainly it could be argued that medical technology in the GFFA should have been able to heal or restore his damaged parts more completely. As a medical professional I do suspend disbelief in this area because some of the injuries described, namely the burns and the heat damaged lungs, rarely remain in a perpetual state of limbo as depicted. In Earth reality Vader's injuries would have either led to his death within a few weeks, or through grafting and transplant technology he would have been healed to a state that would not require the degree of life support shown in the films.
However, without the suit, Vader is not Vader.The suit is life support, but also a prison and concealer. It prevents him from performing the essential human processes of touching and being touched, of eating in a normal social manner, and of expressing himself naturally through nonverbal signals. It is a constant reminder of his physical vulnerability, primarily his inability to oxygenate his body under normal atmospheric conditions, and thus his inability to function without the suit except under very controlled conditions. It is also a literal mask that hides and protects him from the intrusions of others by presenting a fearsome, impenetrable visage, which he without a doubt uses for his own purposes.
In summation the suit effectively isolates him from normal human contact and interaction. Given that before his injuries he was shown to be a highly emotional individual who was greatly dependent on the approval of others, probably no more appropriate punishment could have designed for him than the suit.Whatever one thinks about the idea of Vader's redemption upon saving Luke from the Emperor, it is inarguable that Vader did suffer for his crimes soon after their commission, and for a duration (twenty-five years) commonly associated with "life" sentences on Earth. All things of value were taken from him, and yet he persisted, evidence of his great will to survive.
The suit then introduces the primary drivers in Vader's life : isolation and suffering, with the resulting need for great endurance. A suitless Vader feels none of these pressures and becomes an entirely different person. If one wants to truly understand the character of Vader they must imagine what it would be like for young Anakin to live a lifetime in the suit, rather than contrive a situation in which he is able to avoid it.