We are both talking vbulletin hacks plugins.
Basically what Luciano and I both have commented is this can not be reproduced on a default install with no hacks/plugins or clean install if you will.
The problem with vbulletin to some degree with all the addons and mods out there is that there are conflicts and unwanted behaviors when you run alot of these things. This addon doesnt play well with this and that one does play well with this other one etc.
The issue as it is being seen here is the userid variable which should be empty under all but a few urls is being used or defined by some plugin/hack thus triggering the problem