Re: 4x4 trails in NH or Mass?
I'm familliar with Vietnam.. It is a great trail for mountain biking. In the good old days, it was a great spot for four wheeling, now no motorized vehicles are alowed. Other great spots in mass (that are now mostly gone) is "the pipeline", an area near the former fort devens in Hudson, and an area called the "stow pit" that is now some sort of airport... I remeber when you could 4 wheel in the sand dunes of the national seashore on Cape Cod..
There are spots that are still open. the most technical are in western mass, there's some great spots in and near Monson, Ma. And in and around Holyoke Mass. Get hooked up with a 4X4 club in your area. A guy I knew in Somerville was big into off roading and was very active in his club, and was involved in very technical off roading, his club traveled everywhere in New England.