Vietnamization would have worked had Congress not defunded South Vietnam's armed forces.
That is similar to how the Russians lost Afghanistan.
Former British ambassador to MoscowRodric Braithwaite wrote that the Soviets military rebuilt and constructed hundreds of schools, technical colleges, over 30 hospitals and a similar number of nursery schools, some 400 apartment buildings and 35 mosques, and built a modern infrastructure. They were guarding military and civilian installations. As long as the Afghan puppet government was getting support they remained in power. Sound familiar? After the collapse of the Soviet Union, Afghanistan government no longer had any outside support. After a series of uprisings the Taliban rose to power and all improvements the Soviets provided went to waste. It wont take the 1st Century thugs long to dismantle current Western efforts to make their lives easier.