Inter-process Rendering with VK_KHR_external_memory
The VK_KHR_external_memory, VK_KHR_external_semaphore and VK_KHR_external_fence Vulkan extensions enable the sharing of images/framebuffers and synchronization primitives between processes, allowing one to route the output of one process into another for display, further processing or compositing akin to Syphon (on macOS).
The aforementioned extensions also enable interop between Vulkan and other rendering APIs.
I looked into this for making a compositor and host process for VR development, as SteamVR wastes a lot of time during application startup/shutdown. This technology can also facilitate sandboxing of applications during development as it protects the host process in the event of a crash.