Vermont is well-known for its long history of Republican voting before turning to the Democrats from 1992 to today. Looking deeper, Vermont was indeed dark red from the late 19th to the early 20th century. However, in the 1920s Vermont already began becoming less Republican as the northwestern counties of Chittenden (Burlington), Franklin, and Grand Isle became light red to light blue. This area voted for Franklin Roosevelt even as he lost the state all four times he ran.
Vermont’s most dramatic swing leftward actually did not happen in the 1990s. It happened in 1964 when Vermont swung sharply from Richard Nixon (R) by 17% in 1960 to Lyndon Johnson (D) by 33% in 1964, a 50-point swing. (Even the 1988-1992 swing pales in comparison; George H. W. Bush (R) by 3.5% to Bill Clinton (D) by 16%.) This sharp left turn paved the way for Patrick Leahy (D) to win a United States Senate seat in 1974, and later for Bernie Sanders (I) to win the House seat in 1990.
Vermont was swingy in the 1960s, 1970s, and 1980s, though actually began trending Democratic in 1976. In the 1980s and 1990s liberals from New York and elsewhere in New England moved there. The 1990s saw the transition of Vermont from a swing state to a Democratic state, and the Democratic trend mostly continued to this day. Essex County, the northeasternmost county, is the most conservative, being the only county to vote for George W. Bush (R) and Donald Trump (R).
Here are the results in table form.
Here are the results in map form.