If you just want to play Xvid/Divx in Vista Media Centre, have you tried installing FFDShow?
See here: http://www.ehomeupgrade.com/2007/02/07/ffdshow-media-control-plug-in-for-windows-vista-mce-released/
and http://sourceforge.net/projects/ffdshow
If you want them to display in DVD Library, though, that won't help, and I'm not sure there is a way. Have you tried the My Movies plugin to manage your DVDs / videos instead? http://www.mymovies.dk/