Here are a lot of things that came out of the Vietnam war:
What a short sighted answer the other poster gave. I can't immediately identify 20 good things, but here are a few...
Surgical Glue - google it. The inventor of Super Glue, a.k.a. Krazy Glue delivered it to the US as a surgical glue to close wounds and since then has saved thousands of lives.
An attitude to war - compared to Hiroshima where the world gained an attitude against nuclear weapons, the Vietnam War, particularly for US people have gained an attitude to avoid war.
Helicopter Warfare - introduced in Vietnam for fly in/fly out soldiers has made warfare more tactical and precise and has reduced loss of life in battle.
Halt to Chinese aggression - The VN war dramatically slowed down and in some cases halted Chinese military aggression and occupation. If not for the allied support of South VN, China would almost certainly invaded VN.
Re-think and reluctance to use biological weapons. - Agent Orange (herbicide 245-T) widely used in VN, is now a banned herbicide globally and given the ongoing damage it has caused created a situation of attitude against poisons in warfare.
Recognition of PTSD (post traumatic stress disorder) - previously called shell shock, is now a recognised condition and treatments now discovered/evolved.
Understanding of the "Golden hour". This is where the 1 hour immediately after being wounded is the most vital, if you can get someone to medical care in that hour, their chances of survival increase radically.
Voting age reduced to 18
