Since you used 2 different drives, more than likely the vista drive doesn't have a MBR at all. If you remove the XP drive then use the vista installation disc then it will rewrite it and set it up on that drive.

The xp drive will have to be corrected as well because it will contain vista references and keep asking for dual boot options. These options will need to be removed.

If it was me...I would save all data that I wanted and then do a seperate restore/reinstallation on both drives.