Hmm...well I guess we have to go back to Dave's question, what are you trying to do exactly? Unfortunately there doesn't seem to be any hook when a session object is created except the one you're using, and as I mentioned above, $vbulletin isn't ready yet at that point. I don't see why what you have above wouldn't work (you'd really want to have all the code except the "global $lowlevelundocheck;" inside the if, but that's just for efficiency). However, there are a couple of other places where session objects are created (login and logout) that happen after the global_setup_complete hook, and I don't know how those affect what you're doing.

I should have let Dave handle it, but I thought your problem might just be the global $vbulletin.

I gather you did some standard debugging. As I said in another thread: If something does not work, start simple.

Have you tried setting your variable to something else than true, then printing the variable in the second plugin, to see if it is set?
Have you tried an echo statement in your if clause, to see if the code is being run at all?

Stuff like that.