Ok let's start by saying in the code example you made a small mistake.

-           set the Primary and Secondary Cameras to the Cardboard left and right cameras:

VuforiaBehaviour.Instance.PrimaryCamera = cardboardCameraLeft;

VuforiaBehaviour.Instance.PrimaryCamera = cardboardCameraRight;

^uh, we don't want to set the primarycamera twice. So change it to SecondaryCamera

Now that that is out of the way, when I use your example, I can only switch to mono once and stereo once. After that it doesn't work, nothing happens. So what's the problem?