Never been to VT, but have done and led alot of trips in NW Maine. In Sept, I bet you'll get some snow at some point and probably have wild temp. extremes (summer today, winter tonight kind of things).
I would pre-plan for all kinds of weather, and wait as long as possible to pack until you see a forecast for the time you will be out... I bring a lighter quilt but always bring my BPL puffy suit at this time of year. I may hike in my fivefingers, but I bring thick wool socks to sleep in.
And then be willing to chuck the forecast because New England notches and ridges can sort of make their own weather.
On the bright side, you shouldn't have to worry about bugs! Hope this helps!