Bah, the force is overrated. What is it that Vader can do? I may be wrong, but isn't telekinesis and a low level of telepathy all he's capable of? And how strong his he? Above peak human? And as seen in Episode 2 & 3 he's got some uber martial arts skills.
Sephiroth on the other hand turned an entire sky dark with a mere flick of his hand. Summoning lightning and something that seemed like small tornados in the process. He can fly, has amazing reflexes and is way faster, cuts through tons of concrete and steel in a split second. A lightsaber needs a couple of minutes to cut through a steel door. And Sephiroth's strength is worlds above Vader's. We all saw how Cloud was smashing the dragon Bahamut down with his sword and sliced him apart. And Sephiroth his giving him trouble.
Sephiroth is clearly underestimated. And Vader overrated, just because he has force choke and a lightsaber. Whoop-di-doo.