It's important to remember that this was never a first-choice treatment for seizures. VNS was only viewed as a worthwhile course to pursue after trying an exhaustive list of drugs.
From an FDA hearing on VNS: "there were patients who were very good responders, becoming essentially seizure-free, as well as patients who were poor responders, having 100 percent or greater increase in their seizure frequency," and it was very difficult to predict how a given patient would respond. [1]
From an FDA hearing on VNS: "there were patients who were very good responders, becoming essentially seizure-free, as well as patients who were poor responders, having 100 percent or greater increase in their seizure frequency," and it was very difficult to predict how a given patient would respond. [1]
[1] http://www.fda.gov/ohrms/dockets/AC/97/transcpt/3299t1.pdf p127