The game is not supported yet, it’s out for merely 12hrs.
However, it should work in 2D simply by starting it from Steam. Rift as secondary, “Show only on Rift DK2” in the vorpX config app works just fine here. As always setting the game to 1920×1080 beforehand is important.
Someone on Reddit mentioned he had to start the game windowed and then move the game window to the Rift and switch to fullscreen afterwards. None of that is necessary here though, but maybe that helps in your case.
Irrespective of possible starting issues I would strongly suggest to be a lttle bit patient. You probably won’t regret it, a profile is being worked on right now.
Coming (Very) Soon