On android if your phone is in portrait (vr disabled) and you enter cardboard vr mode it will switch to landscape or at least display the stereo view correctly.
On iOS it will distort the stereo view - you must hold the phone in landscape and then enter vr.
You can fix this by forcing landscape in code first, but this is a discrepancy between android and iOS which other people may well waste a lot of time on if they are combining a normal app with a vr mode as we are.