I am a 16 year old Vietnamese student. Here is what I think. I am not a credible source, but I have to speak. I learned a bit about the Vietnam War, and thought of some ways for the South Vietnamese to win the war. As Mike said, it was impossible to win because we (US and South Vietnam) did not have the support (both the Americans and the Vietnamese). In US, the anti-war movement during the Vietnam was the biggest in US history, Congress did not want to support the war. In Vietnam, the Vietnamese did not like the American soldiers; they looked at them as invaders, just like the French. Neither US nor South Vietnam seek the support of the Vietnamese, the main focus should be to win the trust of the Vietnamese. At the time, North Vietnam, the Viet Cong sneeked into villages and told amazing stories to win their support, though many of what they told were not true. Another factor the war was lost was the Vietnamese government, corruption. Thieu was unpopular. He was Christian and so he pressed the Buddhist.
One way we win that war: If we fight an offensive war. What we did was defensive, containing communism. We waited for them to attack and block. If we striked them, we could have won, we have the power. We could have obliterated their source in Hanoi. I hope my opinions are supported