The Canon books I've read (Vader: Backstories and RotJ: Beware the Power of the Dark Side) seem to have Luke edging out Vader in the early-fight and dominating him in the late-fight.
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."
Well considering Yoda nor Ben tried to stop Luke unlike in ESB when confronting Vader, that's telling how they feel for Luke. Yoda also pretty much said Lukes final test before becoming a Jedi Knight is to defeat Vader before he died.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
As I see it, it's less a matter of Vader not trying and more a matter of mentality having a huge impact on the performance of Force users. A Jedi of average ability with conviction and focus can overcome a more talented one who lacks those qualities. I attribute Vader's loss more to factors like doubt, shame, distraction, and general imbalance than I do any lack of graveness or effort.
I've always heard that Vader was holding back in his ROTJ fight with Luke, but I don't recall ever seeing a single quote to support that... The only quotes I've ever seen have been about Luke holding back against him, while Vader seems quite willing to kill him if he won't turn.
Him turning back to the Light was unexpected not even Ben nor Yoda expectde that to happen, Ben even says he's irredeemable. Luke was trying to turn him, but I doubt his words affected Vader especially since 2 times he could have killed Luke if Luke didn't react at the last second. Lukes just that good, it's head canon that Vader lost to Luke via circumstances.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Huh? Luke states that he sense Vader is conflicted prior to their duel and that there is still good in him. And we see this is true when Vader turns at the end. This isn’t “head canon” it’s presented to us in the film itself.
That Vader may be willing to kill Luke does not mean he is not conflicted. He is. And the film not only states this but shows us this.
And I'm completely unsure why Ben's opinion on Vader "redeemability" is in anyway relevant. He was wrong. Blatantly so.
Vader was not willing to kill Luke during the fight. He hesitates("...")and only decides he's willing to kill Luke after the fight's essentially over(Luke's flipped to the baclony) and even then Luke calls him out on his bluff so I doubt he was even willing to kill Luke then.
People keep misinterpreting this quote:
At this point, when the fight's essentially over, Vader hesitates before deciding he's willing to kill Luke. And even then, it's pretty clear Vader's lying to himself regarding this conviction of his because: https://www.youtube.com/watch?v=U1MnMA0TzGI
In canon Luke is legitimately = or slightly superior to Vader.
In Legends Luke was rage amped for the whole fight (to the point where he basically can be compared to Anakin's final state vs Dooku). When he snaps out of it conflicted Vader beats him back rather thoroughly then Luke goes rampant at the end.
Luke had an advantage whenever he was drawing on his anger, and when he didn't it was more or less a draw despite him fighting defensively. He then proceeded to annihilate Vader with a rage amp.
Also, IIRC sources outright tell us Luke's prowess with a blade is a match for Vader's.