It's much simpler than anyone is saying - Vermont was always liberal. When the liberal wing stopped having any sort of influence in the national party, Vermont went Democrat fast.
There was a period of conservative Republican dominance between roughly the 1880s and the 1940s, fading beginning in the 1930s and in some quarters continuing into the 1950s, roughly coinciding with and not unrelated to the ascendancy of the Proctor family (Redfield, Fletcher, Redfield Jr., and Mortimer, all Vermont Marble Company executives and Governors of Vermont and in the first Redfield's case also longtime Senator and US Secretary of War), but even during this time the Vermont Republican Party had a robust liberal wing, much as many of the Solid South state Democratic parties came to develop wings that were more moderate on race or whatever other issue.
