To answer this question, I am going to need some clarification from you: Who did what better? South Vietnam in the 1960's and Afghanistan in the 1980's are two very separate places and time periods with conditions that were so exclusive to their particular time frame. While a layman might look at these two conflicts and see similarities (both a war against insurgents using guerrilla tactics, both ending with them leaving, etc.), the reality is far from it. For example, the opening stage of the invasion of Aghanistan 1979 was conventional as opposed to the American escalation of the Vietnam War which was gradual and took place over 6 or so years. While the rest of the Soviet-Afghan War was asymmetrical, the Vietnam War was far more fluid, involving engagements of conventional and mobile nature. To put it in more conventional terms, your question is like asking: "Who did it better? The British in World War I or the Soviets in World War II?"