How do I restrict vendors from accessing payment plans?
How to restrict vendors from accessing payment plans depends on the multi-vendor plugin that you are using. In general, though, if a user has the role of Admin or Shop Manager, they are able to add, update and delete products.
However, if your multi-vendor plugin allows users with the role of Vendor to add, update and delete products, they would have access to a product's payment plan. To restrict this access, you would have to add a custom snippet that limits these capabilities for just the Payment Plans tab when editing products in the backend.