There is a very specific physiological action you can take that will engage the Vagus, and it has echoes in yogic practices:

Take a deep breath. Tighten your nether regions (ie, sphincter) and lower abdomen, and press the breath downward as though bearing down for a BM. Now gently release the breath. You can usually feel your heartbeat slow after a single one of these.

This is not a complex exercise, it is simple and natural. Doable in almost any circumstance.

Also, chanting and singing are effective.