I live in Vermont, so here's a few thoughts from someone who sees the foliage every year:
1. I'm going to assume you don't care about seeing foliage colors in the city of Boston proper. There's not much color to see there, though the city itself is wonderful to visit any time of the year. But be prepared, Boston hotel rooms are very very expensive, almost all the time. In September or October, getting a namebrand hotel downtown like Sheraton or Doubletree generally will cost $300 a nite. Better hotels like Hilton will be even more expensive. An acceptable-but-not-great hotel will cost AT LEAST $150 a nite.
2. Foliage colors tend to be at their peak in the northeast corner of Vermont and the White Mountains/northern region of New Hampshire the last week of September, maybe very early October, on average. Foliage colors in the Champlain Valley of Vermont, however, don't peak until Columbus Day, even though the Burlington area is very far north...all due to the proximity of the big lake which moderates temperatures.
3. Foliage colors in central Vermont and central NH tend to peak around October 7, but can be as late as October 14.
4. Columbus Day weekend is generally a high color weekend in southern Vermont, NH and central Massachusetts.
5. If there's heavy rain and wind storms, much of your color can disappear overnight. On the flip side, an early frost in mid-September will make colors more spectacular and intense, with more reds. A late frost arriving in October will mean a more muted experience, with lots of yellows.
6. Book your hotel/motel rooms now. The longer you wait, the less availability there will be in these small towns. These places usually are completely sold out pretty early on. You're unlikely to find available rooms in the best locations if you wait until August.
If you have any other questions, feel free to send me a private message.
Cheers, and enjoy your trip.