Maybe this helps, but i'm not sure.
I'm guessing you must have CMS installed.
I know for me, I had to change the filename forum.php to index.php ..and made sure to change it as well in Admin CP settings.
I have my main forum page as my home page.
so with CMS , the default CMS page is index.php.... unlike with previous vbulletin versions where index.php would normally be the main forum page.
Everytime I download a new vBulletin update, I have to manually change forum.php to index.php each time.
This is sooo cool. I'm responding to somebody who types in Portuguese and the english translation came through perfectly on here..
If you have your forums at the root directory (yoursite.com instead of yoursite.com/forums/ )
Then you have to change forum.php to index.php .. and rename index.php to whatever you want. You will not need index.php if you aren't using CMS.
I renamed mine to "index-whatever.php" and I never use it. Its only for CMS if your using it.
Pleae note that vBET does NOT require to change any file names. Please see previous answer and send us requested information. Also please note that one version of vBSEO had bug (I do not know does in yours it is already corrected). Here you will find issue description with vBSEO official quick fix:
vB4 CMS links not working with vBSEO3.5 - vBulletin SEO Forums