So, the thing about Vendors and Privacy, is that its not a value inherent with the managers, but rather, the times when you go to set it private. When you move to place a vendor, or set a structure's privacy, it will do an internal check to see if the building you're in is private, or contains vendors. You'll want to remove the checks in those two functions.
One of which is the SetPrivacy command. Here in line '62' of the SetprivacyCommand.h file, you'll see an if condition checking to see if one of the objects in the house is a vendor. If so, it will not set the house to private.
https://github.com/TheAnswer/Core3/blob/4d1de291562db5c6922b126d9807190beb48c9e5/MMOCoreORB/src/server/zone/objects/creature/commands/SetprivacyCommand.h
Simply remove that check, and you can set a house with vendors in it to private.
For the other way around, setting down vendors in a private house, you want to look into the code of setting down a vendor in the first place. I couldn't find this file based on the quick search I did for you, but its in there somewhere, likely as a session. You'll want to look for something like "Drop Vendor" or "Place Vendor" and find the check to see if the house you're in is private, then remove that check.
Hope this helps!