Here is a small animation to help you place the nerve within us:
Searching for more information, there seemed to be two main applications in the medical field (a) An FDA approved Treatment for depression when "all else has failed" and (b) Treatment for overeating. The common theme here is satisfaction and contentment, rather than happiness and sadness, IMHO.
For the depression application, long story short, this is an FDA approved treatment option, applied usually when most treatments have not worked (Mayo Clinic). The 'aha-moment' for me came when I saw the info-graphic showing how this is achieved - through a pace-maker. The location of the pace-maker in the graphic made it even simpler to understand:
In Sahaja Yoga, the method to achieve balance, harmony and joy is based on awakening the spirit, "self-realization". Sahaja means born within. Born with us is the heart with the spirit within. In it's awakened state the spirit would keep us in joy, which is the characteristic. However there is a disconnect, caused by imbalance which stops the pulsations of the heart from acting on our central nervous system and transferring the joy of living. This blanket of darkness, called ego, when exceeds it's boundries, envelopes the heart and literally becomes the kill-joy.
Sahaja Yoga explicitly connects oneself with one's spirit, bringing the qualities and contributions of the human spirit onto the central nervous system. Before implaning a battery driven vagus-stimulating pacemaker, I would highly recommend trying the option of self-realization - awakening the spirit to reconnect and send those "happy pulses" from the heart to the vagus nerve and eventually the brain.
Disclaimer: I am not a doctor, and I do not play one on this blog!