Basically, Vader and Sideous were the two most powerful Sith ever, or rather George Lucas says they are, so no other Sith could have stood a chance against either.... because George says it.
Personally, I can't see how Vader could have stood a chance against Darth Bane, the first Sith Master after the Rule of Two was implimented. He had orbalisk armor that made him invincible to lightsaber strikes and blaster hits, and he had complete mastery in Force Lightening. Vader, at his best, would have been decimated in seconds.
But, if this fight could have taken place in cannon, Vader would be written by the author to win.
Bullshit? Absolutey it is. But hopefully with Lucas out of the way, future cannon stories can be told about how Vader wasn't the absolute badass he was thought to be, but he survived and dominated only because of his power with the force.