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Including global.php in any script will allow you to use the user system. If you plan to use the user system outwith the vBulletin installation's domain, you'll need to make sure logins are allowed from all possible domains. In addition, you'll need to set the cookie path to the top level of the domain (e.g. example.com instead of example.com/forums).

There are some PHP constants vBulletin uses to override certain time-consuming functions, such as caching global templates and running cron jobs. You can take a look at the non-standard vBulletin files such as external.php to see these being used.