This never really bothered me and I thought this made sense. Vader is weaker in the Force because well, he really is more machine than man. All his limbs are mechanical and he's being kept alive by a machine. His connection to the Force, while still very powerful, has been diminished by this. Maul for that matter should be weaker in the Force. His entire lower body was replaced by a machine and he doesn't even have the ability to produce life anymore.