Well, the thing is, Vader only asked for Redemption because of Luke. If he hadn't interfered, Vader would still be doing evil and awesome things. And Luke only had to interfere for a few moments. It was only when Vader saw the Emperor torturing his son that he had second thoughts about being evil.

Any time before that, Vader was awesome. Any time after that, he couldn't fight anyway.

The other thing is when he was Darth Haden Christianson, (i.e. before episode 4). Darth Haden Christianson couldn't hold a candle to Sephiroth. Mostly because at that point he's strewn with emotions: hatred, guilt, fear, jealousy, and of course, lust of power.

That makes him an entirely different person to deal with. Specifically, one that Sephiroth could easily manipulate and destroy.

Of course, if Sephiroth first disposed of the Emperor, which he could likely do pretty easily, Vader might actually join him. Or just kill him anyway and join sides with Luke.

But totally, Darth Vader could easily smash Sephiroth, but Darth Haden Christianson would have his butt handed to him faster than Obi-Wan Kenobi on crack.