Vermont was a newly declared Republic on July 8th 1777, when she became the first government in what would become the United State to ban slavery.
The new Republic, which would become the fourteenth state in 1791, was formed after breaking away from New York. The founders met in Windsor, Vermont and drafted a constitution which abolished slavery in the first chapter.
The relevant section is Chapter I, subtitled "A DECLARATION OF THE RIGHTS OF THE INHABITANTS OF THE STATE OF VERMONT"
I. THAT all men are born equally free and independent, and have certain natural, inherent and unalienable rights, amongst which are the enjoying and defending life and liberty; acquiring, possessing and protecting property, and pursuing and obtaining happiness and safety. Therefore, no male person, born in this country, or brought from over sea, ought to be holden by law, to serve any person, as a servant, slave or apprentice, after he arrives to the age of twenty-one Years, nor female, in like manner, after she arrives to the age of eighteen years, unless they are bound by their own consent, after they arrive to such age, or bound by law, for the payment of debts, damages, fines, costs, or the like.