@wayne:
At this point you probably no longer want to go with drupal 5.
Right now, for a production site, I would recommend 6.
(I'm also currently developing a site in drupal 6)
In terms of integration if you got the drupal route, you might be better off using the drupal forums, just for the level of integration you would get. There are tips on migrating stuff from xoops here
http://drupal.org/node/63796searching around shows scripts other people have made too.
http://drupal.org/search/apachesolr_search/xoopsmight be an idea to install a base drupal and test it out. I would also test vbulletin too, and decide which works best for you.
Note that if I was going to integrate vbulletin with drupal I would probably use
http://drupal.org/project/drupalvb(integrate vbulletin into drupal and not the other way around).
however such single sign in solutions are not as pretty as using the integrated forum, and if you can get your data to drupal, then surely that would be the best option (as you do seem keen to have the flexibility of a cms like drupal)