vbulletin is a bit odd compared to other CMS setups, all the files apart from the images are in the database. There are no tpl files or template files as such like you might expect with other software. To edit your theme you have to either use special software or copy the code from your admin cpanel then edit it in dreamweaver or whatever and paste it back into vbulletin cpanel. There are different template parts like header, footer, postbit etc. The easiest things I find is to find a theme close to what you want, install it and make edits on that.