The suit he is in probably does a lot more then help his breathing also. It probably is an air purifier also to clean the air he breaths, and also probably has more features.
Consider the technology in star wars times. Mechanical limbs that function as well as real limbs, death stars, lightsabers, etc... The suit isn't what is keeping him alive. The suit just makes him regain the strength he had before the accident and makes him able to rule the empire still. Vader is a lot like Grevious in that sense. More machine then metal, but still functional killers.
Think of the suit more as helping him become stronger then he was before through advanced technology, rather then the suit keeping him alive.