The event that most directly contributed to the end of American involvement in Vietnam was the Tet Offensive, as it changed the vision of the American society regarding the war, as it began to ask for the end of the conflict.
For the American people, the Tet Offensive was a complete defeat. Not only the optimistic assertions of their military had been totally wrong, but the communists could enter anywhere in South Vietnam. It had completely broken the feeling of advancing in the race, and allied territory had been violated. All the effort of almost three years of campaign proved useless. For many more than those who thought about it at the beginning of the war, Vietnam was nothing more than a slaughterhouse.
Thus began the protests against the war, which would end up influencing the government to withdraw the troops.
