You don’t need to add MSFS to steam. And in fact I don’t think you even can.
You get VD running on your oculus. You should be able to see your PC desktop in the headset. Then just start the sim and when loaded ctrl +tab to get into VR.
I was having similar issues. I thought I’ll take msfs out of the equation. So I tried out a vr steam game demo (from the steam store) just in steam and used VD to play it. That then helped me get the VD working fine without msfs.