Originally posted by Gideon
From Death Star."[...]With that, Obi-Wan stepped in and cut. Vader blocked the attack easily. Obi-Wan attacked again, and again, Vader blocked each strike.
If the old man thought he could rattle him by attacking instead of defending, he was mistaken. Vader riposted, sped up his timing, and took the initiative, forcing the erstwhile Jedi to defend.
He still had some skill, his old Master did, but he was out of practice. Vader could feel it in the Force.
Obi-Wan twirled and blocked a slash, then wove a defensive pattern with his blade. The Force was still with the old Jedi; he was able to anticipate Vader's strikes and block or parry them. But after a quick exchange, Vader felt the energy shift in his favor. "Your powers are weak, old man."
[...]
Vader shoved, hard, and they broke the clash. Obi-Wan retreated a step.
Vader felt the fierce anticipation of victory pound in his heart. "You should not have come back," he told the Jedi.
Another exchange -- four, five, six attacks and blocks -- and Vader knew the old man was weakening. The Force might be strong in Obi-Wan, but the dark side was stronger in Vader. It let him anticipate his adversary's strikes and counter them almost before they began.
Obi-Wan knew it, too. He began to retreat, backing away, his lightsaber itself seeming weaker as he moved.
[...]
But just as he was ready to deliver the final strike, Obi-Wan managed a fast series of attacks, and Vader had to move quickly to avoid the strikes. Even as old and weak as Obi-Wan was, his technique was accomplished enough that a foolish move on Vader's part could still be fatal."
I'm not certain if it was being argued that Kenobi's swordsmanship at this point is superior to Vader's, but the passage from Death Star and all available evidence doesn't conclude as much. The fact that Vader may not have been able to penetrate Obi-Wan Kenobi's defense is not a sign that he is the inferior duelist. This passage indicates quite clearly that while Kenobi held his own quite well, the fight was inexorably heading in Vader's favor.
When reading literature, one must take into account what perspective the writing is intended to be read in. Otherwise, I've found that the default assumption is that there is a narrator with unlimited omniscience. From what you've provided (I've never read DS), Death Star is written in a closed third-person viewpoint. I've noticed quite a few SW novels are written that way.
What that means is that the story is actually being told from the perspective of the characters; being able to jump from character to character in its views. According to Wookiepedia (a dubious source at best), the novel does follow the events from several characters.
So, what's said can't be taken at face value given that we're reading it from a character who's a fallible source. I'm only saying that in regards to your point about the upper hand falling towards Vader.
Anyways, to clear the point up to you: it wasn't that Vader might be "inferior" to Ben Kenobi, it's that he wasn't the clear superior by any means. This was a man who had been out of practice for nearly two decades and who's physical attributes atrophied during his exile. As said, Vader held every card in his favor, but still failed to come near to overcoming Kenobi.
Originally posted by Darth Martin
Vader hasn't lost his skill, just his mobility.
Except for the direct declaration from the official site, using George Lucas' words, stating that his injuries "severely hampered his lightsaber prowess", and the fact that he got overwhelmed by a greenhorn Luke Skywalker with maybe only two months -possibly even as little as two weeks- of formal training. The same Luke, who in the same answer on the Q&A of RotJ (in regards to the question: "How could Luke beat Vader?"😉, that isn't "as skilled or athletic as the young Jedi of the prequels" and who swings his lightsaber around like he overdosed on a bottle of Flintstone's vitamins with a baseball bat in his hands.
No, it's demonstrably evident that his skills have lessened and by a significant margin. Prior to being mechanized, he was capable of disarming and directly breaking the guard of Count Dooku in less than thirty seconds once they fought one-on-one. He also was shown to be far more skilled than RotS Kenobi, even when being under extreme emotional distress (diminishing his skills and power greatly as a Jedi thrives on their ability to keep mental clarity - the same goes for Sith in terms of knowing who and what they are), he held the upper hand.
The Vader you're talking about couldn't even beat a much shittier version of Obi-Wan (in terms of skills, as it goes to badassery - Old Ben > Obi-Wan).