First go to FTP and rename index.php to forum.php
Now it depends where you want your users to go when they click on the forum logo and the forum homepage link in the footer. If you want them to go to the new page you're making then you are done... If you want these links to take them to the forum home that used to be index.php then you need to edit the address of the link in the URL in your header template and if you choose the homepage link in vbulletin options -> Site Name / URL / Contact Details...
Just be sure you have am index.php page ready to upload. Have you given any thought to this?
www.vbadvanced.com CMPS is a free popular choice for making home pages and gives detailed instructions for making it index.php and renaming the old file forum.php.