How did Vermont go from being the most Republican state to the most Democratic

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Invasion of liberals from NY and MA

This is indeed a big factor. However, since the "invasion" largely happened a generation or two ago, Vermont's population cannot be divided neatly into locals vs. foreigners like is often stated.

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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.

This is about right, except the two parties never "switched sides" on race

You've been corrected on that many times.

Do you know how simplistic and moronic it sounds to actually assert that two major political parties "switched" on anything at all?  LOL.

Democrats openly exploited the racial animosity in the South - without apology - from the party's foundation until the early 1920s.  Republicans openly advocated for curtailing that racial animosity - without apology - until the late 1800s.  By the 1920s, Blacks were INCREDIBLY frustrated with GOP leadership (many Black newspaper polls had findings of a slim majority of Blacks actually backing open segregationist Woodrow Wilson...), and all it took to dump the Party of Lincoln was some economic hardship that the Democrats were seen as fixers for.

Republicans in the 1930s felt ridiculously betrayed by the Black community, constantly alleging that the Democrats had "bought and paid for" Black votes, and that the Black community still "owed" the GOP for its actions in the 1800s.  With the Great Depression and World War II taking center stage politically, both the GOP and the Democrats paid literally no attention to civil rights from the 1930s until the late 1950s.  After Republicans saw THEIR civil rights laws being passed against Southern Democratic opposition and THEIR President enforcing Brown v. Board and they STILL lost the Black vote to Stevenson and his openly segregationist runningmate, Republicans (probably correctly) assumed that the Black vote was a lost cause.  Democrats tried to play both sides well into the 1970s, despite the myth that they just had this "coming to" moment in the 1960s where they were now the party of the moral high ground.

I highly suggest you check out the book "Republicans and Race: The GOP's Frayed Relationship With African-Americans, 1944-1970."  It's a good read and very informative.

If you want to simplify it down to a "switch," all you can really say is that yes, now Blacks vote Democrat and Southern Whites vote Republican ... anything beyond that that involves the term "switch" is utterly sophomoric.

The point is, the Democrats were the more racist party before, and now the Republicans are the more racist party. So in that sense, they did switch sides.

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Many of them did go to New Hampshire, Maine, and Upstate New York, which is part of the reason those areas tend to vote Democratic. I'm not really sure why Vermont got so many, though. Maybe because it's equidistant from Boston and New York, so it got a lot of people from both of those places?

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Interesting video that relates to this subject.

https://www.youtube.com/watch?v=06xR1uxp5mI

Made by a conservative think tank, so it should be taken with a grain of salt.

Edit: also it's largely funded by the Koch brothers, so it's kind of hilarious that they're complaining about other groups having out-of-state funding.