Modification on permission control

[mjiang]

Anyone could give me some suggestion about that?

I have several subforums and what I want to do is allow
only specific user group to visit the respective forum based on
the some custom info for that user.

Ex. there is a custom field "Account type " for any user in our
system, so how to forbid thoses user which has some special "Account type" cannot access one of my sub forum?

Is there any way to implement this in the interface of vbullitin,
if not, I wanna know where the source code we should change
and how to locate it.(Note, Actully I tried this in the interface of "search template and style" but not found).

[Dismounted]

This requires a modification and cannot be done via default vBulletin features.

[mjiang]

Anybody know where I can find the code for link to sub forum in index page?