Originally posted by Major Valerian
He lost his potential to become the most powerful Force-user ever, but that does not mean he could not become more powerful. He was no-where near his peak by ROTS. Don't forget that even after losing critical parts of his body, he still was 80% of the most powerful Sith Lord.By ANH, he has been training under Darth Sidious for [B]twenty
years. He has obviously progressed in terms of force mastery, at the very least. [/B]
He never really did train. He couldn't even conjure Sith Lightning without hurting himself. Sidious despised the fact that Vader was unable to learn Sith arts and never really taught him anything. You can think of that relationship as that of Starkiller and Palpatine in the Ultimate Sith Edition of The Force Unleashed videogame.
I think Sidious did teach Vader Sith knowledge. Outside of the lightning disability, he was very much taught to channel his rage and such in the Sith ways and probably got most of what he could use. Iirc Jerec, one of the dark jedi who defected was jealous of Vader for having access to said Sith knowledge, but even in Vader's shape he was still considered a better apprentice.