i don't think you can disable the in-car mirrors. You could move the virtual mirror over the spot of the in-car mirror to block it so to say if that helps.
Not really, I only need the virtual mirror. The in-car mirrors are just using up resources to render them needlessly. I test drove a car a few days ago that had a rear-facing camera in addition to a regular rear view mirror, and along with the driver side wing mirror and the virtual mirror there were four mirrors (with different FOVs) on screen! Is this a design decision or a game engine limitation?
They are all the same image, so additional mirrors don't take up resources to render. Technically, maybe a few microseconds to duplicate, resize, crop and insert them in the right place on the car.