Many of the roads are in valleys... especially north-south roads... for example, Rte 5 runs north-south along the Connecticut River on the eastern edge of Vermont and is pretty flat and not very busy. There are also a number of rail trails, but unfortunately at this time they are a pretty short... however, the Lamoille Valley Rail Trail will be 93 miles from end-to-end when fully built out.... but now it is only a 15 mile section from St. Johnsbury to Danville and a 17 mile section from Morristown to Cambridge.
Route 2 from Barre to Burlington parallels the Winooski river and is relatively flat.
Also, the western edge of the state along Lake Champlain is pretty flat... as is Grand isle county which include the big islands in Lake Champlain. There is a nice rail trail along the eastern shore of Lake Champlain in Chittenden County that is pretty flat.