Well, as this place proves, migrating from xoops to vbulletin is certainly doable. There are some bits you'll have to get your hands dirty doing, if the experience here is anything to go by:
1) Private Messages aren't imported.
2) Links to the old site content threads aren't updated.
3) If you use the myalbum module, you'll need to find a replacement.
4) If you use the news module, you'll need to find a replacement.
The chances are, any replacements for (3) and (4) might not have any automatic way to import from XOOPS.
Basically, you need to roll your own import code if any of the above are important to your site.
