We prefer a convex mirror in vehicles because it gives a wider field view.
Convex mirrors form a virtual, erect and diminished image generally. This gives a wider and better view compared to plain or concave mirrors. When a vehicle behind us is far from us, then the image is formed at the focus of the convex mirror and as the vehicle moves towards us the virtual image also moves towards the mirror giving a realistic view of vehicle movement.
Due to all this, convex mirrors are preferred in vehicles.