Regional/Protestant identity politics. The Vermont GOP governors of the 1930s were some of the most left-wing in the nation at the time.
I think the "left-wing" or "liberal" or "progressive" nature of Vermont's past Republicans - even those from the Aiken Wing - tends to be exaggerated. Yes, it was a near-one-party state, and that one party was going to include genuine liberals and progressives ... but it comprised a lot of (arguably way more) genuine moderates and genuine conservatives, as well. I think your average liberal Republican from Vermont back in the day would be a lot closer to Charlie Baker than he would be to Bernie Sanders with an R next to his name. I tried to do a little research on these governors, though:
Jon Weeks (R) - I can't find much on him, either, but he was a small business owner farming and selling insurance, and then he served on the board of two banks. I think it's at least a decent bet that he was a moderate.
Stanley Wilson (R) - He was responsible for implementing a state income tax, but he said himself that the positive effects of this were getting rid of several other taxes, including the highway tax and multiple property taxes. Again, I would say it is fine to claim he was no conservative, but I don't know if he would be standing arm and arm with liberal Democrats.
Charles Smith (R) - I can't find much on him, and he only served one two-year term.
George David Aiken (R) - He was definitely a center to center-left Republican, but he wasn't exactly "left-wing" by any stretch. While accepting much of the New Deal, he certainly battled the aspects that didn't help Vermont, which - IMO - was a key aspect of Vermont's brand of conservatism: they were fine with some "progressive" initiatives that would help their community (state) and were controlled by their community (state), but they didn't particularly care for federalized progressivism that imposed on their autonomy. It can't be argued, however, that he was the namesake for the Aiken-Gibson Wing of the Vermont GOP, which was certainly the more liberal and progressive wing of the state party.
I would say all four were clearly center or left-of-center from what I could find, but I bet there were governors who were decidedly more left-wing. In fact, I would argue a lot of Vermont's "progressivism" was decidedly NOT left-wing in that most of it was designed to help Vermonters - a largely independent, rural, White and Protestant bunch. Also, it needs to be noted that the GOP during the Great Depression was simply playing the role of the right side of an Overton Window that had shifted dramatically left following the stock market crash. Many Republicans who carried a conservative philosophy compared to their Democratic rivals - Dewey, Willkie, Landon, etc. - wound up sounding like moderate liberals because that is the only space they could really occupy.
However, back to the OP, I do agree that the liberal and progressive Republicans who were a part of the Vermont GOP were, indeed, a part of it due to regional and Protestant identity politics. As NC Yankee said, there are a lot of very interesting and informative threads about why the state changed, which is an infinitely more interesting question that can't be chalked up to "parties sorted ideologically, and Vermont was always liberal." (I'm not saying anyone here is suggesting the latter.)
