Can the vagus nerve cycle between being overactive and under active? what would be the symptoms?

Yes.: Overactive example pass out when see blood. Sympathetic failure? Trigger points localized points discomfort with bizarre symptoms if activated are sympathetically maintained. There is sympathetic sudomotor syndrome where compress area lets say neck muscle and only 1/2 forehead sweats. Simple injury to sympathetics can produce severest of pains that may destroy persons life called regionalpreviousn.