Motion controller inputs and tracking are supported by the built-in OpenXR plugin, so Varjo OpenXR plugin is not required to enable controllers in Unreal.
The Unreal-provided Virtual Reality Template is a good starting point for a project with motion controllers. Unreal Engine 4.27 comes with a completely new VR template designed for OpenXR, which provides basic VR interaction and locomotion.
Varjo OpenXR runtime currently supports Vive and Index controller input mappings. You bind action mappings for these in Project Settings > Input > Bindings.
Refer to Unreal Engine documentation for more information on how to set up and use the motion controllers.