Does the vest feel uncomfortable under your suit?
Keeping those kidneys warm is key to staying warm in the water. Before I bought my 4/3 hooded Xcel, I had an old 4/3 Psycho. Id stick hand warmers on my back, neck and in my boots. Definately helped.
Not trying to sound like a total bad-arse, though I definitely am one, but I'm not really seeing why a heated vest is necessary. If you're trying to stretch a 4/3 into winter suit then maybe, but man...a good 5/4 gets you in the water comfortably when the air is 10F and the water is 35F. Seems like more of a luxury thing, which is fine.