Try to reset the view with ALT+SPACE. The viewport is supposed to be a little bit asymmetrical though, vorpX (correctly) centers the virtual screen to your eyes, not the center of the headset viewport.
All VR headsets render more downwards than upwards as a slight optimization: human vision is tuned to concentrate more on things below your eyes than above, so headset vendors figured it would make sense to utilize this psychological effect and render more pixels where it counts (downwards) and less where it’s less important (upwards).
Hence a certain asymmetry with slightly more space visible below the virtual screen than above is perfectly correct when the screen is centered to your eyes. For some headsets more than for others.