There should be a readme that came with your purchase of vBulletin. First thing's first, you need to make sure that you have made/imported a database for vBulletin to use. Next, you need to make sure that you have Intermediate PHP. If you don't have it or just want to check, it can be found at
http://www.x10hosting.com/account at the bottom of the page. Once all of this is checked, you will need to run the install file for the forum, which should be at something like http:// *your site url*/*path to vbulletin*/admin/install.php. I haven't done a vBulletin install in ages, but the readme that should've come with the software, or the vBulletin site should help you right out.