Using vBulletin as master for single sign on (SSO) ?

[yellowpinky]

Hi,
I would like to use the vBulletin username / password as the master list for single sign on.

That is, the user logs into vBulletin, forum.myexample.com
User clicks on a link to another custom application, myapp.myexample.com

Somehow, there is a root cookie(?) that myapp can retrieve to know the authenticated vB username ?

Example 2: user goes directly to myapp.myexample.com
user was not logged in, so they get redirected to forum.myexample.com to login
then they come back to myapp and are now authenticated with a vB username.

Is there some documentation or tips on using vBulletin as the master database for SSO?
Or is it better to use some add-on product. vbPortal (don't know what that is, but the name makes it sound like a possibility).

Thank you for any assistance.