Correcting camera matrix to use with GearVR.

I'm developing for GearVR, which unfortunately is not supported using Vuforia 6.5

I have created my own application with camera feed with VR mode enabled (Oculus SDK). However, one major issue is a distorted camera image in each eye

(The rectangles are supposed to be a square)

Trying out Vuforia i see it is possible to counter this distortion, and i was wonder if it is possible to replicate the calibration steps using Vuforia in my own application.

(Vuforia's camera is correcting the aspect ratio of the camera)

I've been thinking i need to take my byte[] stream and use a custom matrix multiplication in order to fix it, but i have no concrete idea on how to do this.

How is this camera calibration done in vuforia? The documentation does not explain it.