Have you ever seen a dog die from heartworms? It's horrible.
I have not doubt. But it doesn't change the fact that there are no mosquitos in Vermont in the middle of winter. It would be like vaccinating for a disease that isn't even present in the area.
I always heartworm test in the spring and start the meds before the mosquitos come out, and I keep going until they are safely gone. I would never suggest doing otherwise. For the 15 years we had our Bedlington in Vermont, I never once heard the vet suggest using heartworm year-round. My vet did say that they now "recommend" year-round heartworm, but when I said I only wanted to do mosquito season, they didn't blink.