As a separate country, any relations would be, by definition, voluntary...on both sides. I assume that since they would be seceding, they would also be renouncing US citizenship. They are landlocked, and nothing would stop the US from putting onerous border policies into place. All highways leading into Vermont could be converted to toll only. There's those Federal programs such as SS, etc...cut them off.
You put it very well. I might actually like to see it happen. Sometimes very foolish people need very hard lessons.