I was able to get WOFF UE to work with VIVE. Head tracking with the headset works fine. Only problem was that when I used "geometry" setting in Vorpx, there would be a lot of screen flicker even after disabling shadows, so I had to choose "Z-Normal".
Used Opentrack with Vorpx. Simple setup. Didn't need to use any virtual desktop either, Vorpx just hooked into cfs3.exe when it was launching.