As DJB noted, the Rt 100 corridor is a good choice, I’ve ridden a lot of it, and like all Vt roads, there’s not much shoulder, but drivers are used to cyclists so give you room. One nice thing about VT., is there are country stores at like every Hwy, junction, so getting food and water is easy.
The route though is dictated by where you stay. Camping in state parks means the route is park to park for the distance you choose to do each day. Waterbury to Brattleboro via Rts 100 & 30 is 135 miles with a campground at Camp Plymouth in Tyson at mile 75 or so. But what are your plans for daily mileage. Hard to recommend anything else without that info.