Route from Bar Harbor, Maine to Essex Junction, Vt

Sounds like you're on the right track.

Where are you going in Essex Junction? If to the fairgrounds ("Champlain Valley Exposition"), I would suggest getting off I-89 at Richmond and taking 117 (River Road) into Essex Junction, continuing on to 15 at the "five corners." That's the only potential place I know of to camp in Essex junction...though perhaps you're visiting friends or have some other plans.