I beleive it was answered above in the 3rd post about your prefix.
Your cookie prefix is not set correctly.
You can view the config.php of vbulletin and see what its set to by default it is bb
I stated the issue that your forum cookie path was not right
Now you can use phpmyadmin like I explained in post 5 to set this if this is post install or you can reinstall and enter the correct cookie prefix if you are not familiar with phpmyadmin.