as an earlier poster said, I wouldn't worry too much about lining up your trip with a specific event. There is enough to see and do in northern New England in the summer that you will be able to have fun anytime June through August. I grew up in NH and lived in VT as an adult, and still go back to visit family and friends a couple times a year.

I would definitely recommend going to a fair. Addison County would be fine if you are near there, but there are many others in VT and NH and so you just need to find one near the time you are there. I would recommend some of the smaller ones that still retain a local character and have less commercial shows and midway rides (but they all have enough rides and booths to entertain kids). Go to the 4-H cattle and sheep shows and the horse-pull competitions, etc. Cornish, NH (about 15 miles south of where I-89 crosses from VT into NH) has a classic small fair - usually mid to late August [oops - just saw that might be too late for you - but there are other fairs]

Drive through the Green Mountains in VT on Route 100 or through the White Mountains on the Kancamagus Highway in NH. Stop and go tubing somewhere along the way. Go to Maine and take a boat ride day trip to a small island off the coast.

VT, NH, and southern Maine are small enough areas that you can take day trips across much of them.

relax and have a great time.