There is a branch of the vagus nerve that innervates the inner portion of the outer ear. This branch is called the "Alderman's nerve" or "Arnold's nerve". The vagus nerve, actually heavily used in our system from the ST 10, ST 11, ST 12 area more than auricular-wise which we use sparingly, carries sensory information from the physical organs in the body and elsewhere. The vagus contributes to the control of heart rate, blood pressure, stress, gastrointestinal issues, sweating, speech, breathing, and more... We tend to use the points above as we feel it is a more direct approach to stimulate the vagus with our tools, there are, of course, many ways to accomplish this.
Auricular acupuncture is a microsystem of acupuncture. Microsystems are systems focused on one area (the ear, certain parts of the hand, certain parts of the feet, etc.) where you can access and treat illness anywhere in the body. Technically you can treat the majority of conditions treatable by acupuncture by just using ear points although most acupuncturists either focus primarily on body points which can be more direct or use a combination. The possibility of auricular points reaching throughout the body may very well be due in part to this branch of the vagus nerve. There are, however, points in the ear which create changes that have nothing to do with the functions of the vagus nerve - yet that doesn't mean that the messages don't initially travel via that route.