How do I make Vendor(s) non-Global?

It is best practice to try this operation in your Staging Environment before undertaking it in Production. See this link for how to access your Galley Staging Environment.

Directly associate your selected Vendors with the appropriate Locations by following the steps below.

Enter the Location.
Choose the Source tab.
Hit "+" and explicitly choose the Global vendor by clicking on it.
Say "No" to the "make this vendor non-global" dialog.

Now you can go to each of those Global vendors and safely make them non-global because only existing Vendor Item-to-Ingredient associations at all Locations that are global will be disassociated -- not those that are now not directly associated with this Vendor.

Once you've practiced this with a Vendor or two in your Staging Environment and are satisfied with the results, you may perform these steps in your Production environment.