Fake Vivi is a combatant in the Simulated Twilight Town's Struggle tournament, although he turns out to be a Dusk disguised as Vivi. You must combat Fake Vivi once more as Roxas while searching for the truth behind the Seven Wonders of Twilight Town.
Kingdom Hearts II
A Dusk takes the form of Vivi in the Simulated Twilight Town. It takes out Seifer in the Struggle match, and reverts to its true form once Roxas beats it.
After a glitch of multiple Vivis, the fake appears before Roxas, asking if he was practicing for the Struggle. After leaving Roxas's eyesight, it reverts into a Dusk and flies off.
Kingdom Hearts II
This battle takes place during the Struggle Tournament on Day 4. this battle may be the most difficult as the Fake Vivi is much faster and more relentless than any of the other competitors. Be sure Roxas has the Guard ability equipped to block the majority of his attacks. Fortunately his attacks are not powerful. Quickly gather up the orbs before Vivi collects them. During the battle, Vivi will close the distance more often than the others leaving little room for Roxas to run. In addition, look out for powerful overhead swings where Vivi enlarges his bat. The last major attack to look out for is a rapid spinning Vortex-like attack where Fake Vivi spins with his bat through to air towards Roxas.
His one weakness is the duration of his attacks. Keep circling him and when he attacks, counterattack from the side or behind. If speed is an issue, use the Guard ability and counterattack with a combo. The key is to keep moving, otherwise he will attack Roxas and gather up orbs very quickly. Just use normal attacks when possible. If all else fails, collect a couple more orbs than Fake Vivi then just run around avoiding his attacks until time runs out.
The second battle takes place in the Tunnel on Day 6. This battle is very easy as the clones Vivi makes only have 1 HP each. Simply use normal attacks and the battle will end fairly quickly.
Notes and references