Now, as someone who has pointed out the demographic changes before, I don't believe that should be interpreted as the sole major reason why Vermont is voting Democratic. In terms of ethnicity French/French Canadian, English, and "American" still make up almost half the population, there are a lot less Jewish people than in neighboring Massachusetts, and Vermont is probably the least Catholic it has been in half a century. What is changing is not only that more people are moving in, but that the people moving in are influencing many of the people (especially younger Vermonters) who are "native" to that state. I believe this would explain, for instance, the 34% of the population that reports as having "No Religion".
But anyway, Vermont has been and always will be an odd duck. Back in the early days of the Republic when the rest of New England was heavily Federalist Vermont had a tendency to be lean DR. Hell, Vermont even elected people who would have no business being a dog catcher in Massachusetts much less an elected US Congressman at the time.
Yes, this is an underrated reason for Vermont's (and the surrounding area's) political change.
