For me, the amazing thing about the Vagus nerve is that you can stimulate it with your breathing. And since the Vagus nerve affects the autonomic nervous system (involuntary, not consciously controlled) while breathing choices are conscious, this is one of the few ways that humans can directly impact important aspects of their psychology and physiology, like how calm they feel. Mindfulness + breath choices + vagus nerve => altered state. And that's why traditions including yoga and buddhism empathize breathing. Couple great books that touch on this are:
Breath: The New Science of a Lost Art https://www.amazon.com/Breath-New-Science-Lost-Art/dp/073521...
and
Altered Traits: Science Reveals How Meditation Changes Your Brain https://www.amazon.com/Altered-Traits-Science-Reveals-Medita...