Thanks for the answer, but i did not help.
I tried everything now for FSX:
Started SteamVR first, second VorpX, third FSX.
Started only VorpX, then FSX.
Switched between DX9 and DX10.
Changed resolution.
Started FSX via the Steam library (i have the non-Steam version).
Edit: OFC the Vive is in direct mode and i also tried starting VorpX as administrator.
Nothing worked. Either i have a black screen on both devices, or i have the game screen on my monitor but on the Vive i still have the VR background that shows up when you turn SteamVR on.