This book from a viet cong founding member is pretty good, they had support from the north and some direction, but as time went on the north became significantly more radical and crazy. By the time of unification he had to escape because the north were crazy ass fuck.
It’s called a Viet Cong Memoir. Sorry I misremebered.
Vietnam, Cambodia, Laos all fell to Communism, despite military efforts. The most informative piece that I read while writing my dissertation on Vietnam: http://www.amazon.com/Vietcong-Memoir-Account-Vietnam-Aftermath/dp/0394743091