DarthSlinky mentioned "his personal canon" and I agree with this. I think all of us Star Wars fans have our own personal canon. We take bits and pieces of Star Wars lore from the movies, novels and games and piece together our own lore that makes sense to us. Personally, I believe that the force resides within the spirit, not from Midi-chlorians and yet Lucas himself created that concept. I remember Grand Master Yoda telling Luke that we are luminous beings not this crude matter. That along with the spirit of Jedi returning to the force tells me that our spirit is the true connection to the force and not bio-matter. Others would disagree.
This is why I believe that Vader's power and potential remained the same before and after his injuries. As long as his spirit stays with his body his power will remain unchanged. I believe that Lord Sidious referred to Vader as a shadow of his former self because he never fully embraced the dark side as he started too when he first fell. Anakin Skywalker was always there regretting what he had done within Lord Vader. His dark side potential was never reached. Embracing the dark side and being a dark jedi is not the same thing.