US draftees served for only a nominal 2 years which, (IMHO, and I was there) was one of the primary reasons why US forces in Vietnam were less effective than they could have been. They came in to the Army, received about 6 months of training, went to Vietnam, and then often got out several months early. Another was the US policy of individual replacements...that is, soldiers joined a unit that was already in-country and went home after 12 months. The result of this was that any given infantry squad of (theoretically) 10 men usually had 2 vacancies, 2 people who were new and hadn't integrated with their comrades and 2 who were within 90 days of the end of their tour and had nothing much else on their minds.

It should be remembered that the overall US military strategy in Vietnam was designed by civilians (which might or might not have been appropriate), but so was the concept of operations. Whether or not the outcome would have been different if the soldiers had been allowed to fight the war is a question that will never be answered. The same is true in Iraq.