I got this working when on Win 8.1 64bit (sillyly upgraded to Win10 and no joy now!). My setup is a GTX980 GPU if that makes a difference.
Steps that worked for me:
1. Set the Rift to be in Extended mode.
2. Start Vorpx
3. Turn off Rift
4. Launch GTA V with Rift off
5. Get loading screen, wait!
6. When white splash screen is up and only then turn Rift on after a few seconds.
7. Win! :)
If turning it on too early get the same error as the OP, is all about timing it right. Also sometimes the vorpx doesn’t hook in, all about the timing so keep trying.
Game is great in VR, especially driving :)