Have you copied the file config.php from vbulletin, or by any chance made an empty one?
If you copied it, then it should work and I'm not sure what the problem is. Eventually, if you wish, perhaps you can PM me the file to take a look (there might be something changed/broken in its format). Don't post it here, as it probably contains sensitive information from your installation.
In case you made another one, then please do copy the existing file from vbulletin folder /includes, instead.