It's a very progressive state and always has been. Very, very liberal attitudes up there. When the Democrats still struck the electorate as more "traditional", they lost Vermont by large margins every time. Sure, Nixon had the 1968 Southern strategy, but the Democrats had not taken that turn into the "progressive" choice in America. (Actually, Vermont was right around McGovern's national numbers in 1972 after a century-plus of voting WAY more Republican than the nation as a whole, and he was a progressive)
The evangelical Carter wasn't going to facilitate the switch. But Mondale got VT to the nat'l average in 1984, it was D+2 in 1988, and it was al downhill for the GOP in VT from there as the GOP continued to drift rightward and the Dems realized that the liberals and progressives could be incorporated into their coalition.
This doesn't really capture it, though--think the Proctor dynasty or Ralph Flanders. Nothing very progressive about that sort.
Well of course you get some habitual voting and recent Southern New England transplants. I tend to think Vermont was always a place where they prefer the government stay out of your business and, for whatever reason, they don't interpret taxes and healthcare mandate as too much of an intrusion... but those social issues and things like the Patriot Act cross the line.
And reading up on Flanders... I don't see anything so out there (for his time) in his platform that would dissuade Vermonters from voting like they had for over a century. He was vehemently opposed to Communism, an ideology that in the 50s was tied to government control and totalitarianism, which goes to my point that Vermonters like the government to stay away unless it comes to practical matters like taxes and healthcare.
One of the reasons taxes are not an issue is because there isn't much income inequality. As for healthcare, I'm not sure why anyone would consider it an intrusion. I was happy to read the other day that Shumlin will do everything he can to bring universal healthcare to Vermont.