Vulkan (instance and surface) was not implemented in the platform plugin when Vulkan support got introduced in Qt 5.10.
This should be remedied for 6.0 because Linux users will now more and more often run Qt applications as Wayland clients, and may want to utilize Vulkan for Qt Quick rendering.
Note that this has nothing to do with the compositors' use of OpenGL. This is only about the wayland platform plugin. In a proper Wayland system with support for both OpenGL and Vulkan the clients can render with Vulkan even though the compositor may use OpenGL to do the compositing.