Well is it that hard to be a top tier duelist in the OT, when Vader's only had like what....6 duels? 1 of which was against Koth who gave up completely on being a Jedi, so I doubt he's any better than what he once had been and would probably be weaker.
Tbh, I would say Vader's skill neither increased nor decreased. Far as I can tell, in Canon he's not having many actual duels.
I mean let's see, the only notable Jedi Master who actually would more than likely be honing his skill would be Kirak. Since...that's what he's stated to be, his whole thing was just fighting.
Koth being the next guy, well as said, the guy basically gave up on being a Jedi, so at BEST his skill would be the same as it was in ROTS....though I really think he wouldn't be as skilled.
The Grand Inquisitor next, I gotta be honest here, I really don't see how skilled this guy actually is when all he's done is face rusty/learning/relearning Jedi Padawans. Like that shouldn't really do much to help Vader other than maintaining what he has.
Next is...Tano. Next to Kirak, she's probably the most skilled. Just on the basis that she didn't seem to give up fighting? I gotta be honest here, the Inquisitors in Canon are just garbage, that I don't see where anyone can see an increase in saber skill for....well anyone.
Then Obi-Wan, again the guy has been out of the game, I seriously doubt his lightsaber skill just magically increased beyond what it was in ROTS.
Reva? She's trash.
So I feel there's more evidence that Vader's skill just didn't go anywhere really, didn't increase, but didn't really deteriorate either. It just kinda remained stagnant.