The same way that your state went from being so Democratic to so Republican. The ideologies of the parties began to shift, especially on racial and other social issues, the states became closer because of the deep partisan tendencies despite ideology changes, the states became very sympathetic to "liberal Republicans" and "conservative Democrats," and by the 90's and 2000's had completed the shift over to the other side.
Vermont may have become more liberal in recent years, but it has always been a liberal state. And keep in mind that being predominately white doesn't matter as much electorally in the northeast as it does in the south.
You've been corrected on that many times.
