speaking from experience, if push comes to shove (which sounds like it may), you could take a “roundabout” approach…
catch a bus from millinocket to portland, me.
then, snag the train line from portland to a point in southern nh (say dover, durham, exeter or portmsouth). from there, hop the bus (concord trailways) to white river jct. and then onto burlington VT.
long, but it’s an idea 
leslie