Press Down, Forward, B. Vegeta should do a sliding Kick. While he is sliding, press Down, Forward, B. Repeat this four times. He should not continue the sliding kick, and will now be using different attacks. At the end of the fourth attack, press Forward, Down, Forward, B. He should do his Knee Slash Attack. If done correctly, it should take off about 100 Power. You can also keep pressing Down, Forward, B rapidly, and at the end press Forward, Down, Forward, B.

Stand next to your opponent, but do not hold Forward while punching or kicking. Kick your opponent, then Punch and press Down, Forward, B four times with timing. At the end of the fourth "Down, Forward, B", press Forward, Down, Forward, B for the Knee Slash Attack. This should bring down your opponent's power to around 239.