Thanks to Garrett A. Wollman for this information, which would have been difficult for a non-Vermonter to deduce from a pile of maps. The route of the highway would have followed the existing I-189 to Shelburne Rd. The full plan was cancelled in the late 70s, when Burlington began to revitalize itself as a tourist destination rather than a manufacturing city. In 1992 it was decided to forgo Federal Interstate funding to appease environmental concerns by only building a 'super two'. Back in the days when highway planners were in their ascendancy, there were a couple of other highways planned for Vermont as well, both of which left some remnants for today.
Unnamed sources are either maps or personal experience.