The Vendor role within kinops is rather limited by definition. Vendors will see their discussions and the Services and Queue kapps when they login.
When they access the Services kapp, they will see only Services that are accessible by “Everyone”. By default, this is only the Feedback item.
When they access the Queue kapp, they will only see items assigned to them personally unless they are in a group, and then they will also see the tasks assigned to that group. You may decide to create a group for a specific vendor, so you can assign work to them as a group and they can assign it within that group.