Yes, I needed to know this myself so I could update it LOL! But vBulletin Subscriptions allows you to give someone access to a certain forum when they have paid for something so in this forum you could have a link to the radio OR try Usergroups > Promotions and when they have reached a certain post they can join (automatically) a new higher group which may have access to the radio.php file (to change access permissions to this) Read this thread (
here as you can see it allows user permissions system as well on the PHP files you simply edit radio.php