If you see ROTJ when Luke takes his mask off, there is a continuity error. In one shot the breathing device is on then the next shot over his sholder Vader is missing it. Cut back to the front and its back on.
But ignoring that small mistake, I also figured that it was always on. Maybe not the full face but at least the breathing apparatis.
I don't think the faceplate was needed for breathing as such, anatomically (sp) that isn't correct. Breathing is controlled through the chest, not through the mouth, that is merely the opening to the tube that carries the air.
In order to act as a respirator, wouldn't he then need some kind of close fitting mouthpiece over his mouth and nose to force air down? Kinda like the idea when the "bag" someone to assist in breathing?
Well, for handicapped people with respirators, a tube is applied in the throat. They cut a hole in the throat, place a tuve with is connected to a breathing apparatus. But I wonder if Vader had such a thing. According to the injuries visible in the ROTJ x-ray shots, all of his neck was replaced by metal. Bet that got pretty beaten up. PLUS the masked was for his vocals as well.