Actually, it would require a very minimal knowledge of PHP and vBulletin's code. All you have to do is do something like this:
PHP Code:
require('/full/path/to/includes/class_vbulletin.php');
$vbulletin = new vBulletin;
Now that code will probably not work (wrote it without actually thinking of file names... But then, while it is running, find any PHP errors / warnings that occur (usually file could not be found at location blah blah), go to that line in the class file and hard code your file paths. Just keep track of these because this makes updating a PITA since you have to add those hard coded file paths manually.
This is why I think a definable FORUM_ROOT variable would be a really really really really nice feature.