I have just read an article ( http://www.edmtunes.com/2016/02/headphones-actually-get-high ) that a company named Nervana ( http://experiencenervana.com/technology-vagus-nerve-stimulation/ ) demonstrated headphones at CES 2016 that can stimulate Vagus and thus increase dopamine, serotonin and oxytocin. It certainly looks interesting on the paper, but I am a bit skeptical about the negative effects which this stimulation could bring.
Let's say that they actually work - how can they stimulate only vagus and not the rest of the nerves which could have unknown effects? Vagus is also connected with a lot of other key functions such as digestive system, heart etc. Wouldn't that possibly have side effects? And the similar goes for hormones, what's up with homeostasis then? If some hormones increase, won't that have consequences at least by a feedback?
Here is an interview with cofounders: http://futurism.com/this-startup-gets-you-high-on-dopamine-no-exercise-required/