I was able to extend the Vendor Table and Page with some custom fields, and expose these via the standard API extension methods in AL. However, using the the standard HTTP PATCH method for Out of the box entities ( https://docs.microsoft.com/en-us/dynamics-nav/fin-graph/api/dynamics_vendor_update ), and using the endpoint for my custom API, I get an "Invalid Request Body" error.
The custom url for the endpoint is of this form: 'api.businesscentral.dynamics.com/.../vendors%{id_of_vendor})'
This is the code for the API extension. Is there special code required for an implementation of PUT or PATCH?