I have this exact same setup (D700 with HTC Vive) for doing VR work, and it does definitely work. You need to install Steam VR through Steam for Mac. Plug it in the HDMI port and one of the USB ports (and power). Run FCPX and select Window->Output to VR Headset, and it should load Steam VR and it should output to your headset.
The ONE catch is that you may want to use an older version of Steam VR, because recently they broke something with playback, and you might get black frame flashes. I did archive this older version that is confirmed to work, so if you want it message me. Basically you have to install Steam VR normally, and then overwrite it with the older version. Also, avoid running Steam or it will auto-update (just activate Steam VR as above from within FCPX). Now, it's possible that they have fixed this bug by now, but I haven't checked lately.
The administrator has disabled public write access.