OK, first you want to go here:
https://www.vbulletin.com/en/customer/account/login/ and log in with your customer number and password. Then under "My Account" in the upper left, click on "Members Area". Then there'll be a list of licenses you own. Under License Options you want to click on one of the download options, depending on what version of vbulletin you have. Then you need to know the exact version you're running, and assuming it's not the latest version, you want to click on the "More Download Options" radio button and select your version from the dropdown list.
Edit: Oh, I'm assuming you don't have vb5. I don't really know much about that.