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.
