Your index.php page is what first loads when you visit a folder location ie:
http://yoursite.com/forums the index page would be first to load.
If you have your site setup, so your forums is in a subfolder, such as yoursite.com/forums there wont be a need to rename that file. Simply upload vBaCMPS into your yoursite.com and rename that cmps_index.php to index.php and call it a day