A work around I think we’ll be able to use is with sub-categories. Here is a example i’m working with:
The layout…
Content Management (parent category, protected access to cm (content management) group)
-Videos (sub category, protected access to cm group and videos group)
-Videos-VENDOR (sub category, protected access to cm group, videos group and videos-VENDOR group)
I change the group permissions for videos-VENDOR to reply, view. Then if a thread in “Videos” warrants vendor access, I move the topic to “Videos-VENDOR” along with any other topic I want vendors to have access to. Then i can change what they can do via their user level.
One issue with the above is that if the vendor doesn’t have access to the parent category, they can view the sub-category items that show up in “Latest” and click on the topic title to interact. However, if they click the sub-category title, they get an error “Oops! That page doesn’t exist or is private.” @codinghorror, so the work around that allows sub-category access without giving parent access might be a security bug-turned-feature 
Although if that loop hole was closed, the better method would be to give the vendor access to the cm group so it had access to the parent, but don’t give them access to the other subcategories.