1. When Should I go? I see dates anywhere from right about now to the third week in October. In my view, when planning a trip, you have to set realistic expectations. While there have been some signs of early color change, the recent warming trend may retard that. I think you have to play the odds and go during the "normal" peak periods, which really are the last 2 weeks of September and the first week of October. In my experience, no matter where you are in the state (with the possible exception of the Champlain Valley), foliage is essentially done.
2. Where Should I go? There is really very little of Vermont that isn't photogenic. But in foliage terms, the "where" is tied to the when. There are locations that are sure to go early and locations that will be later. BUT, I differ from much of the advice given here in this way (and this is not based on anything more than experience and speaking to locals). The conventional wisdom is that foliage progresses from North to South and you are "safer" later in the southern parts of the state. I think this is generally true, but is also overgeneralized. My experience and on-the-ground intel has taught me that things like elevation, and proximity to major geographical phenomena (like mountains, lakes and rivers) play more of a part. For example, My experience has been that Peacham in the Northeast Kingdom often holds its color longer than any other place. At the same time, I have photographed in Waits River and in Newfane and have never really timed the color right in either place. In both places, locals have told me I was too late and that mid-September was "the peak."
3. What is Peak? Good question. It is, in my opinion, completely and utterly subjective. I think it is more important to focus on whether there is color at all (what I might refer to as "good color").
So what to do with this information? There is no better "boot on the ground" source than here and very soon, we will start seeing "real-time" reports. Vermont is a small enough state that you can go a long way if you want, to find the color. Be flexible and willing to move with it, and change your daily plans. Most of all, enjoy what you get. Good, bad or indifferent color, it is a pretty magical experience.
If it sounds too good to be true, its probably . . . .