Seriously why? Although Aqua is the one who transformed Land of Departure into Castle Oblivion, it was Master Eraqus keyblade who actually made it turn into Castle Oblivion. And since it been confirmed that Sora found Eraqus keyblade on the beach of Destiny Islands, Sora can find Ventus himself. All he need to do is sense the connection between the keyblade and Castle Oblivion, go to there, beam a light at the castle, and boom. He finds Ventus. (At first, Sora will think it's Roxas until Mickey tells Sora that's Ventus, and tell him about Aqua and Terra.