Xion and Vanitas have never been seen interacting as Vanitas was destroyed a decade before Xion was even created. Xion is currently inside of Sora while Vanitas seems to exist in some aspect as Sora saw a vision of him.
VanShion is a rather popular Kingdom Hearts ship despite the fact that it is a crack ship. Many fans like the fact that both seem to be rather dark and have matching dark hair. Both are also connected to Sora. Vanitas is the other half of Ventus who took on a similar appearance to Sora due to him and Ven connecting. Xion is a replica of Roxas who eventually became one with him and Sora. Many feel that their personalities would match well.
- Some fans have theorized that Xion might be an incarnation of Vanitas in a similar manner that many believe Roxas to be Ventus.