How did the geographical boundary of Vietnam come about?

When WWII ended, the Allies occupied Vietnam and assigned Great Britain and China to disarm the Japanese. The country was divided at the 16th parallel with China in command of the North and Great Britain in command of the South. In the South, the British helped the French regain power, a power they had held in Indo-China for a century.The Viet Minh, under the leadership of Ho Chi Minh claimed North Vietnam. The French and the Democratic Republic of Vietnam (DRV or North Vietnam) entered into negotiations for a planned free election for national unity. The election never happened. The French and Viet Minh fought a bloody war with a terrible French defeat in the valley fortress of Dien Bien Phu. At a peace conference in 1954 the French and the DRV agreed to a temporary division of the country at the 17th parallel. The French in the South and the DRV communists in the North.At this time, the soon to be famous DMZ or de-militarized zone was created at the dividing line of North and South, the 17th parallel. Once again, an election was planned to vote on national unity, and the election never happened. By the 1960's Americans had totally replaced the French in South Vietnam. The U.S. Central Intelligence Agency established one government and then assassinated the president they established. In 1964, acting on disinformation about an attack upon U.S. warships in the Tonkin Gulf, the U.S. formally attacked North Vietnam and America's longest war was underway.