If Vader did lose power with his limbs, I pose this question. Since Luke lost his hand did he lose any power? I don't believe so. I agree that just the effort of staying alive is sapping so much of his strength that Vader is limited in what he can do. Also we should consider the abilities and limitations of the mechanics of Vader's limbs. Granted prosthetics are much more advanced in the SW universe and no doubt Vader was busy tweaking them, but there still are limitations to what they can do in comparison to organic limbs. I'm wondering if anyone else thought about this, and if I'm wrong just let me know.