I changed my dual-boot (Ubuntu 8.10, Windows Vista Ultimate) Dell XPS M1330 laptop to a single boot machine running Ubuntu 9.04 with Windows Vista in a VirtualBox 3.0.0 VM (using 1GB RAM and 40GB of HDD space).
After installing Guest Additions I have Vista running well at fullscreen. (I tried Integrated mode, but it feels a little messy to me.) As expected, the early Direct3D support is rather slow, but I’m not too concerned. I’m mostly interested in running different browsers and FlashDevelop without the kludge of dual-booting.
I, too, have had similar experience here at work on the similar host and versions – installing Vista is quite easy on virtualbox. Migrating from VMware to virtualbox is a little different but Ive just managed that (http://ubuntuforums.org/showthread.php?t=1209274), and now Im up to converting physical vista installation to virtualbox vm of which I havent tried yet. Good times!