BTW: The same is true of Orange County, although it still leans Republican.
Here's the 1916 electoral map:
Here's the 2000 electoral map:
41 out of 48 states flipped parties between those two elections. So there's nothing remotely strange about Vermont flipping parties. Most of the rest of the country did as well.
Vermont was a Republican Solid South. In addition to traditions, which are not fading so easily, the state Democratic Party as a viable political force is relatively a new thing up there, so lack of alternative was an important factor too.
The Republican Party as a real force in the Deep South is also relatively a new thing. There was a saying that before 1960s, you could have Alabama Republican Convention held in a phone booth.
