To my knowledge at present the only STALKER game that runs in DX11 under VorpX is Call of Pripyat. Other STALKER games – SHOC, CS, LA – run only in DX9 mode (I hope Ralf will implement R3 renderer to LA soon).
If you don’t want to enter game only for changing DX settings, than I recommend you to enter user.ltx in appdata folder. In user.ltx you find a line “renderer renderer_r*”. You must turn it to “renderer renderer_r2”. R2 is DX9 mode.
Makes sure you use the rift in extended mode.
You’ll also have to set the FOV, becouse STALKER games use 75-85 FOV as default, and with the rift you’ll need a much higher FOV, around 120-130. STALKER LA and COP do have fov command from console. SHOC doesn’t, it is rather tricky to alter FOV there, but not impossible. I don’t remember right now how FOV is altered in CS, but surely possible.