I'm going to push back a little bit here. Regardless of what we know about stuttering treatment, the original treatment for TENS / Vitalstim / electrical stimulation of muscular tissue has little to no evidence in speech pathology. Vitalstim is a company that offers a proprietary device for use specifically with swallowing disorders (they only evidence they have comes from swallowing disorders after brain trauma, and that's spotty at best). OP's SLP knows next to nothing (IMHO) about stuttering. Stuttering is not associated with weakness in a particular articulator (this is clear just by observation: when it comes to stuttering is there too much or too little muscle contraction, always too much). While something appears to be working here it's not likely a result of electrical stimulation.