Are you using Windows Media Player or Windows Media Center for playing dvds in Vista Ultimate? Media Center plays dvds seamlessly for me, though I'm not a fan of their DVD Library. It is a pain in the butt to set up and use, though Microsoft does give info on how to set up the blasted thing:
http://support.microsoft.com/kb/930526
I normally use
PowerDVD in my XP Pro partition and VLC works pretty well these days. I use VLC to play ISO image files of dvds.