When I visit the forum using "www.darktrain.org" it doesn't remember my login cookie, and I have to log in every time. If I use just "darktrain.org" it can find my cookie just fine.
I think this only started happening since the upgrade to 3.6.8.
Edit: According to the documentation:
http://www.vbulletin.com/docs/html/m...ons_group_http
I think you want to go
Admin CP -> vBulletin Options -> Cookies and HTTP Header Options and set "Cookie Domain" to .darktrain.org (with a leading .) or change it to blank so the cookie domain will be set automatically.