The diaphragm is also innervated by the Phrenic nerve...c3-c5. In medical school we learned the cool rhyme. C3, c4 , c5 keep the diaphragm alive. Breathing is very helpful. Not only are you controlling the vagus but you are also directly manipulating the diaphragm. Using OMT I can usually stop hiccups. Basically, I press underneath the costal margin and directly manipulate the diaphragm (I press on the diaphragm through the skin and have the patient breathe in and hold their breath. Once I feel the diaphragm relax I tell the patient to exhale. This would indirectly work on the vagus while also influencing the somatic phrenic nerve!