Vader can beat Sephy for the following reasons:
  1. He can choke him with the force while Super Nova is being animated.
  2. He can destroy Sephy's sword.
  3. He can totally confuse the mentally unstable Sephiroth wiht the good ol' "I am your father," line.
  4. Sephiroth may be able to destroy the solar system, but if the Death Star had been finished our galaxy probably wouldn't be here for Sephy to destroy. Nyahhhh.