This project shows one way to embed a media player inside an OpenGL application. In this demo application the LWJGL library is used, but the same approach should work with any general Java OpenGL library.
Support for rendering via OpenGL requires VLC 4.0.0 - this version of VLC is not yet generally available, if you want to use it you must build the latest VLC from its sources, or use a prebuilt development snapshot if you can find one.
You also need to use the vlcj-5.x development branch, OpenGL rendering is not supported in vlcj-4.x (vlcj-4.x uses VLC 3.0.0).