Fiiiiine. Xehanort forcibly extracted darkness from Ventus's heart to take over Ventus's bod because Xehanort is a weird Orochimaru freak. Anyway... doing so it fractured Ventus's heart. Ventus's heart went to Sora, Sora was still a child at this time. Sora helped it become whole again, replacing the darkness. Vanitas originally was faceless but with Sora's connection it made it look like him.