“US out of Vt!” — What a sad, sad end to Vt’s history. Such could only happen at the hands of the weak and the cowardly. Ethan Allen, roll over.
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.
Ironically, it's the mirror image of how Vermont entered the Union.
Vermont had a very interesting history in colonial times. The colonial government of New York gave grants of Vermont land to wealthy absentee landlords (known as patroons in Dutch), while the colonial government of New Hampshire granted the same land to smallholders who actually farmed the land. Ownership of land in those days was determined by who payed the property tax and who accepted payment. New York forced the issue by sending in sheriffs to collect the property tax, and Vermonters resisted by forming citizens' militias. (Two militia leaders, Ethan Allen and John Stark, played important roles in the Revolution.) New York and New Hampshire were in the act of polishing their swords and oiling their muskets to decide the issue when the events at Lexington and Concord intervened. Following that, both colonies agreed to table the issue until the larger problem with Great Britain was resolved.
Following the war, neither state had the money or political will to force the issue, so Vermont by default became an independent republic with its own constitution. It sent no delegates to the Confederation Congress or the Constitutional Convention.
In 1790, Lord Dorchester, the governor-general of Canada, opened negotiations with Vermont in an attempt to bring it into Canada. Before he was knighted, Dorchester had been General Sir Guy Carleton, the only British commander George Washington had truly feared during the late war. Due to British stupidity, Carleton had spent most of the war in Montreal, cooling his heels. Now he began a campaign to strangle the American republic in its crib, and most of his efforts were spent exciting Indian tribes along the border. Vermont was simply another opportunity for Carleton to make trouble.
President Washington looked at the map and understood instinctively that a Canadian Vermont would be a British Vermont, which would be a spear sticking into America with the head of the spear aimed squarely at New York City. Washington tasked Secretary of State Jefferson to write a letter to the government of Vermont giving it an ultimatum. Either it could apply for admission as the 14th state -- or Washington would send the army into Vermont, conquer it, annex it as a territory, and make it a state when he damn well pleased. Faced with this threat, Vermont broke off negotiations with Dorchester and applied for admission.
Power politics can be brutal.