Hi Clif. I have ridden all of it at one time or another. Vermont ain't flat, as the saying goes. If it were ironed out, it would be nearly as big as Oregon. You will seldom ride on the flat. Our glacier-carved terrain is easier going north and south, though. The tough climbs are all east-west.
The scenery is beautiful, of course. It is no accident that organized bicycle touring began in Vermont.
Route 100 is all two-lane road, with shoulders varying from none to adequate. Nowhere are they excellent. South of Warren and north of Morrisville, the route is rural and traffic is light enough to rarely be a problem. Between those towns, however, is major tourist country. Streams of cars and trucks in both directions is the rule during summer and again during foliage time--late September to mid-October. The most-visited tourist attraction in the state is on 100 in Waterbury: Ben & Jerry's ice cream factory. (We do not like to admit this, but there it is.)
Some map study will show you parallel roads for much of the busy part. If you and your bike are ready for some well-maintained unpaved roads, so much the better. 52% of Vermont's public highways are unpaved. The Delorme state atlas is good, although it does not distinguish paved and unpaved roads.
Do you have some special destinations in mind on Route 100? If not, I can recommend a much nicer all-paved alternate around the busy part. Get out your state road map and follow along.
100 north to Stockbridge. 107 to Bethel. 12 to Morrisville. Rejoin 100 and continue north. Do not be alarmed by passing through Montpelier. The smallest capital city in the nation, its population is 8,000. It is a rather sleepy place in the summer, when the legislature is not in session.
Fred