The Vermont Agency of Natural Resources oversee much of the state’s land (including the State Forest and State Parks) and it has a Geocache Policy that covers Letterboxes as well. To plant a Letterbox in a Vermont State Forest or a Vermont State Park you need to summit a Geocache Request Form by email or mail to the address on last page. Geocaching and Letterboxing activities are not be permitted within state wildlife management areas or fishing access areas.

http://www.vtstateparks.com/pdfs/geocache_policy.pdf
http://www.vtstateparks.com/pdfs/geocache_request.pdf