Vermont was a very "Yankee" state for a long time (centuries, actually), which meant heavily moralistic Protestantism and the kind of reformist public life and politics that followed from that. There was more than a tinge of anti-Catholicism in this Protestant moralism, in addition to being virulently opposed to Southern slavery/rebellion, or anything that sought (or was perceived as such) to undermine the Union. In that sense, Vermont and other strongholds of Yankee/New England Protestantism - perhaps more than anywhere else - identified their own values as being equivalent to the nation's values.
Today, Vermont is considerably more pluralistic and secularized than what it used to be, with plenty of Catholics, Jews, and other types who, in the 21st New England context, are very liberal (and who the remaining Protestants have accommodated, even as New England Protestantism has changed in many ways to be more pluralistic and less "evangelical" 
), but it still retains a lot of that core Yankee/New England Protestant moralistic cultural influence on its politics. It's the interactions of Vermont's long-established cultural traditions with the added influx of newcomers that makes it, in contemporary terms, one of the most "liberal" places in the country.
Lol I see someone else read
American Nations.
But yeah, this post nails it. It was a combination of New Englanders becoming more Democrat-friendly across the entire region along with an influx of almost exclusively liberal outsiders. Without them, it'd probably be more like New Hampshire politically, but they've helped tip the balance and make Vermont vote heavily Democrat for national elections in line with the rest of the "Birkenstock Belt" (VT + Northeast NY + Western MA and parts of Western NH).
