The story:
We installed the MODx <-> vBulletin cross-login plugin some weeks ago but found it unreliable. At least the login on the MODx side did not always work (to be honest, it almost didn’t work at all). We removed the login fields and will tell users to login via the vBulletin forum which acts as a master for this plugin.
The situation:
But we need at least the "background" functionality for custom PHP because we have restricted areas in the MODx part of the website. The forum is the main part of the site (registering there is required to do anything). So far the cross-login seems to work but I found some non-reproducible situations where I couldn’t get the user with PHP from the $_SESSION array although he was logged in.
The question:
Is anybody here using this cross-login addon and can tell me if it is reliable?
I expect a loud "YES, reliable."

(There is very little time left to the launch of the site and if the cross-login doesn’t always work I would not know how to get the current vBulletin user on MODx pages ...)