You can install vista over opensuse (I'm not sure if vista requires to be installed on the first partition!). During vista installation it will overwrite the Master Boot Record (that's the first sector of a hard disk which is responsible for booting), so that for a moment you'll only be able to boot vista, however you can easily restore ubuntu's grub loader using
ubuntu's install cd or supergrub cd
http://www.supergrubdisk.org/