The correct answer is "South Vietnam fell".
Vietnamization was a policy implemented by President Richard Nixon to progresively terminate with the participation of the US in the Vietnam War. The program goals were to equip and train South Vietnam forces so that they would take the leading role in the combats, releasing US troops from those responsibilities.
In the end the program was a failure as it could not prevent the fall of Saigon, the capital of South Vientam, in 1975, and the subsequent unification of North and South Vietnam for the creation of the Socialist Republic of Vietnam.