Ok, to do this you need to have a portal installed.
The preferred portals that integrate with vBulletin are:
vBAdvanced
vBPortal - does not integrate with vBPlaza!
MKPortal
When you have selected your portal, you need to follow the instructions for use that come included with it to install it.
If you use vBAdvanced, you will need to rename the cmps_index.php file to index.php
Once it is installed and configured to your likeings, remove any existing front pages that you have on your server, and your portal will be open.