I use vBulletin at one of our sites, at http://theparacast.com/forums and have gone through the foo of importing our posts into a new database from another host. So I think I have enough war stories in me to cope with your situation, if you need some advice.
The instructions provided with the software, and at Jelsoft's site, should also help you. If not, as a registered user of vBulletin, you are entitled to their support as well, to guide you through the process.
Take it from me, since my Webmaster left us high and dry on an extended vacation that hasn't ended yet, and that is that none of this is hard. Just follow all the directions and pace yourself, and you'll survive
Co-Host, The Paracast
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