In a nutshell, tDCS modulates spontaneous neuron activity by increasing or decreasing the thresholds that determine whether the neurons send a signal. The relatively local effects of tDCS can spread throughout the brain by networks of interconnecting neuron circuits.

There are some forms of vagal stimulation you could try: gagging, holding your breath while bearing down (Valsalva maneuver), immersing your face in ice-cold water (stimulating the diving reflex), and coughing. I'm not sure whether these would have a lasting effect. Sympathetic blocks (injecting lidocaine or bupivicaine) can help reduce sympathetic activity; though they are most effective regionally (placed near the sympathetic ganglion chain), some patients respond to i.v. (total body) infusions. My wife's sympathetic issues always improved with ECT. There are also more permanent but invasive approaches (implanting a stimulator).