Here are a couple of links for you:
http://www.vbulletin.com/docs/html/main/upgrade
http://www.siteground.com/tutorials/...in-upgrade.htm
I'm doing a similar upgrade. Upgrading vbulletin itself is not a big deal. It's your hacks that can take time. I have a lot of custom work on my site and so I am having to write my own products/plugins for the upgraded site.
Edit: Oh, some other advice. Create a test site (you can do this if you htaccess protect it) and do the upgrade first on there to see what kinds of issues you may encounter.