From Bulbapedia, the community-driven Pokémon encyclopedia.
Once the Liberty Pass is obtained, the player must take it to Liberty Pier, the westernmost pier of the [[Castelia City]] harbor. The captain of the boat docked here will recognize the Liberty Pass, and volunteer to take the player to Liberty Garden. If the player does not have the Liberty Pass, the vessel remains stationary.
However,if Victini is traded,you can't get it back; the only way to get it back is start a new game.