the cookie file would be called somthing like "username@site.com" in ur cookies directory - C:\Documents and Settings\username\Cookies
1. Get an app that can modify cookies
this is a nice free one but there are probly many others
http://members.lycos.co.uk/nirsoft1/utils/iecookies.html2. copy the cookie file from victims computer (or use XSS)
3. change the username to urs (i think that should work) or
can modify ur own cookie (assuming u have logged in to the site)
simply substitute ur username, id and password hash for the ones contained in the other cookie file u have "aquired" - this method has been tested on invision, xmbforum and phpbb so VBulletin should be no different
Note:
if u have grabbed someone elses cookie it may have expired, but that is no problem cause u can change the expiration date as well from that nirsoft app
a cookie file could also be stolen using cross site scripting (XSS), eg:
that is a cut down version of one method i have seen where the cookie is sent to a script that adds it to a database