I'm a dork
 

um. I am trying to set up vbulletin on a site... and usually the forum urls can go http://domain.com/forum .. but I know they can also go http://forum.domain.com .. when I try to set up mine for forum.domain.com it does not work? is there something I should be doing?

you got to go into your CPanel of the host of the domain and set up a subdomain

I've actually done that and its still not working.

when you do that, it usually makes a folder of that name... so it should work

...although setting up custom image locations and cookies makes it get messy depending on the complexity of your installation...