I think the Democratic trend in the 80s had a lot to do with the fact that the older generations (primarily Yankee and Republican) were dying off and making way for the younger generations who were more Democrat. That's all I really have to say on the subject.
This, and more liberal out-of-staters moving into the state in large numbers.
I don't think Vermont has ever had people move there in large numbers.
Certainly enough moved in to affect the state's demographics. In the early 2000's it helped spawn the Take Back Vermont Movement:
http://en.wikipedia.org/wiki/Take_Back_Vermont
It's always been 3 electoral votes though so the population hasn't changed much.
Electoral votes are allocated to the states based on their population relative to the country as a whole. Between 1980 and 2000, the state gained nearly 100,000 new residents, which was a roughly 20 percent increase in its population size. There's plenty of room for out-of-staters in there. Because it's such a small state, an influx of out-of-state transplants is going to have a more noticeable effect.
Also, the state has not always had three electoral votes. It had two districts until 1932, and at one point had as many as six districts.
All I'm saying is that the population Vermont has gained doesn't compare to most other states. You're saying the same thing as me basically. Vermont can grow a lot and still not gain electoral votes because other states will always have a higher percentage of the nation's population. Many people moved there, but not as much as other states. I'm sure it didn't ALWAYS have 3 electoral votes, but in the last several elections it's stayed the same.
