In the end, I transitioned the entire thing with about 1 hour of downtime and essentially no loss in functionality. The only things I didn't migrate were reputation (deliberate) and classifieds (would have been too much work).
I'm incredibly pleased with the ease of management of XenForo as compared to vBulletin. I was also very worried about how the members would take it - they tend to really, really dislike change. Polls and discussions seem to indicate the majority are happy with it, though!
Check it out, let me know what you guys think.