Answer
Use a separate 'go-between' middleware service that produces or consumes the actual flat files on the third-party side, and have that go-between service communicate with the IFS instance over standard RESTful web-service calls made over HTTPS instead of file drop-zones. A properly configured HTTPS-based service integration doesn't require a VPN at all, sidestepping the whole file-drop-zone/per-partner-VPN problem that direct file-share-based integration would otherwise require in a cloud-managed environment.