Comments and user authentication

hi
I’d like to allow/disallow users to make comments on my blog based on a condition they are actively registered and logged in to a VBulletin website – one I host say. I was thinking some kind of cookie authentication.

if ( !isset($_COOKIE["VB_userid"])) {
 // You need to be logged in to VBulletin to post comments.
 // Disable comments form.
} else { // Show comments form}

It’s a starting point although I doubt if it will be this simple in real terms. Could you give me any pointers as where to start.