The vagus nerve wanders all through our bodies and controls all of our automatic functions like breath, heart beat, digestion, elimination... It is a key survival mechanism as well, stimulating the nervous system to react in case of danger either by arousing the body into action or shutting down if action isn't possible.
In 2012, Stephen Porges developed "Polyvagal Theory"—a groundbreaking way of understanding how the vagus nerve works. He and Deborah Dana describe automatic vagal responses as a hierarchy—not the simple back and forth shift from rest/digest to fight/flight/freeze. Here is a link to a presentation by Deb Dana.
Understanding how the vagus nerve works is a first step toward "reprogramming" it so that we are not triggered into automatic reactions. Instead, we can learn to expand the energetic container of the body so that we can hold difficult emotions rather than get triggered by them.