If they need your cookie salt, that's it. It shouldn't matter what the value is, as long as you use whatever vBulletin uses. If you want the exact (scalar, and not variable) value, then open up your config.php file and find
PHP Code:
$config['Misc']['cookie_security_hash'] = 'XXXXXXXXXXXX';
The cookie salt is supposed to be your vBulletin license #, which you can usually find at the top of all your files.
PS, probably not a good idea to post your license #.