Switching between mixed reality modes on android using Unity

I've been searching online for the answer to this for quite a while and haven't found a solution though I know plenty of others are asking this question, perhaps I am not searching for the right things or in the right places.

Basically, I have a mixed ar/vr app for android built in unity using vuforia and I want the app to start in AR where a scene is spawned on an image target and after the user gazes at one of the objects in the scene for a set amount of time it loads a VR scene.

The AR scene works perfectly fine and so do the object triggers but when the new VR scene is triggered I cannot seem to get VR to work correctly, most of the time the scene is just black or black with the button for going back to ar moving around appropriately but still the VR background is not visible.

I tried adding a gameobject to the scene that changes the mixed reality mode to handheld to vr but this doesn't seem to work:

MixedRealityController.Instance.SetMode(MixedRealityController.Mode.HANDHELD_VR);

What else do I need to do to successfully transition to VR? There doesn't seem to be much documentation on how to actually make the transition happen except that you can use the line of code I pasted above. I've looked carefully at the vrar sample but that is for transitioning in one scene and I'm not sure how it changes between scenes as I don't know the order in which or how vuforia initializes in regards to scene and camera etc.

I would make it all one scene and imitate the vrar sample transition but the scene is already really heavy so I'd like to keep them as separate scenes. 

Also, it works with Cardboard enabled but I want a handheld none-stereo VR experience and there doesn't seem to be a clean or easy way to disable stereo mode for cardboard.

Do I need to:

    disable background video randering upon loading the scene?

    disable the datasets?

    and then switch the mode with: 

MixedRealityController.Instance.SetMode(MixedRealityController.Mode.HANDHELD_VR)              ?

    The documentation seems to imply that the above code handles all that automatically. Do I need to destroy or do anything with the camera or should any or all of this occur with a 'void awake()' method so that it happens before the camera is initialized?

I will keep experimenting but any help or tips would be greatly appreciated and I could really use a better overall understanding of how vuforia works and initializes

[using unity 2017.3.0f3 and vuforia 7]

Thanks!

Graham