In your admin control panel, under styles & Templates, edit the footer template. Simply add the analytics code to the bottom of there. http://www.vbulletin.com/forum/showthread.php?t=199652
Yep, that will work! Just to elaborate, your footer is included in every page of your vb site, therefore your analytics code is good if someone enters your site from any page other than the homepage. But, if you are running a separate homepage, not vba, then you will have to run analytics in that also. Hope this clears your fuzzy head!