I think the issue still comes down to the OT, though, even taking the PT into account. Why? Well... Think of it this way, Vader must have at least twenty years' experience more than Luke, even putting aside training time. ROTJ establishes this by noting that Luke and Leia were hidden from their father when they were born. Since Anakin was Obi-Wan's student before that -- before he turned to evil -- that must give him at least the entirety of Luke's lifetime as greater experience. Plus, Vader helped "hunt down and destroy the Jedi knights" so he's been in far more duels/combat than Luke has. The training itself, I would argue, isn't the issue. For me, I interpret it as this -- Vader in ESB is testing Luke and his raw power. He's not really going full strength and Luke never stood a chance. In desperation, he got in one lucky hit and then Vader cut off his hand to quickly end the duel. In ROTJ, though, Vader was conflicted and (more importantly) didn't want to kill Luke due to the residual of Anakin resurfacing. That's really how I think Luke overpowered him -- he was in a rage, such that Vader would have had to kill him to stop him, but Anakin didn't want to. Looking at it this way, I don't have any problems with the presentation of the duels in the OT.