The quickest and easiest way to get an offline test forum is to use something like xampp
http://www.apachefriends.org/en/xampp-windows.html
What this will do is install and configure everything you need to host the vBulletin software. This includes (but is not limited to):
- Apache (the webserver)
- PHP Engine
- MySQL (the database)
Once you have this installed, you can then take a backup of your live database and import it into your test DB, additionally copy the VB files from live to test.