One forum (vbulletin) for multiple sites / multiple domains

Hi,

The network appears to be stable (after fixing the www & non-www) issue for the main blog but now I'm thinking about the next step.

main.com has vbulletin in a subdirectory

Network is subdomains:
subdomain.main.com --> newdomain.com
subdomain2.main.com --> anotherdomain.com
Etc

I'd like to have these forums available to all subdomains. Does anyone have a suggestion on the best approach?

Should I link to just the main category for the subdomain?
Should I link to the front end of the vbulletin?

Does anyone have a link to maybe a sample of someone doing something like this ....

Thank you.