My project is very simple. I have around 1500 registered subscribers on a Wordpress site, in the root of the domain. On a subfolder (/forum) I have a licensed VBulletin forum setup. I don't want to sync the users, all I need is for the /forum folder to be protected and only be accessible by registered blog users. I want them to have the ability to register for the forum, but nobody else.
Essentially, if anyone navigates to the /forum folder, I want something that checks if they are logged into the blog. If they are, the forum pages load normally, if they are not logged in, they are sent to a blog login/join page.