Displaying an AR scene inside a VR-headset (Unity, Vuforia, Oculus Rift)

Hello,
can somebody please help me with an issue concerning Unity, Vuforia and a VR-headset?

I have created an AR scene with Unity and Vuforia which works fine so far.
My goal is to display the AR scene inside a VR-headset. So I attached a webcam in front of a Oculus Rift and enabled the virtual reality support in the build settings.
The problem I am facing is, that in VR the 3D-object isn't mapped on the marker correctly anymore but is hovering above.
Does somebody know, how to solve this problem and to diplay a AR-scene in a VR-headset? I am very thankful for any help.