Here's how I understand it, to make a VR app which can display one or more of your physical monitors you "just" need to make an app that's a fancy upgrade to VNC, nothing trivial but you'll be building on some fairly solid ground.

To make a VR app where programs all just float around in space you need to build more like a whole Window System like X11 or Wayland, which is a decent chunk of an entire OS.