I think I see. Perhaps this will work.
Create a group “Vendors” lock that group at TL0 or TL1. When a Vendor joins, lock the user at TL0 or TL1. Give TL0 read/reply access to the Vender Agenda category.
But you’ll still need to remove everyone from the trust levels of all the categories.
Make the TL1 (or TL2) permissions such that your normal users won’t be locked out for long.