Well, I decided to try again and did a fresh install. The same issue with vsync happened again. (Older versions of vorpx didn’t have this issue.)
Anyway, I tried with a different graphics card (GeForce 1060) and that fixed the issue but this means I’m losing out on freesync. Annoying.
Some issue remains with R9 390 + vorpx – hopefully the next release will clear this up.