[Logician]

I cant think of an very easy way but if you are determined to apply this (and you are good at PHP and vb login algorithm), you can try this method:

Change the default vb login behaviour in ami-boards.com and when someone logins in that board, open 2 windows (1 window with 2 frames might work too) and assign relevant cookie in amidreaming.org too by triggering a custom script in amidreaming.org.

For example: When I login to vbulletin.com, vbulletin.com assigns me its cookie. But if programmed, it can also trigger a script in a say popup window (or frame) in vbulletin.org and make sure vbulletin.org will assign me its cookie as well. So when I visit vbulletin.org, you can read my cookie since its assigned by vbulletin.org.

Hope this helps a bit..