Having the correct light source in the best position will eliminate shadows on your face.
Vertical lights on both sides of the mirror will provide the most even illumination. This can be achieved with wall mounted lights or with pendants suspended from the ceiling.
Horizontal fixtures with multiple lights placed over the mirror can also provide even illumination. Fixtures above the vanity should be at least 24" wide and the center should be mounted 75"-80" from the floor.
Tip: Recessed lights over a bathroom sink can cause unflattering shadows on the face.