vorpX uses the mechanisms that the headsets provide for that. Both the Oculus API and OpenVR have explicit calls to send them the rendered image and then do the screen sync themselves.
The vorpX FPS counter isn’t 100% precise though. If you get this exact same behavior in many games, it’s probably just the FPS counter not displaying the 100% correct value.
The value you should optimize for is not the “Direct Mode FPS” but the “Game FPS” value. That’s the impprtant one. Ideally it should also show the full headset framerate. That will be seldomly de the case though. The lower the Game FPS is, the more likely it is that the end result isn’t 100% smooth. Try to reach at least 50fps.