I am sceptical about those percentages, but even if they are valid there are also valid qualifications.
"Volunteers" would include people about to be drafted (when they saw their low number in the draft lottery) to try to avoid being in the combat infantry and, presumably, people who 're-upped' when a draftee's original tour of duty ended.
The general level of education of the whole nation had improved. At the time of WWII a high school diploma was almost the equivalent of what a college degree was becoming at the time of the war in Vietnam.
On the ultimate scale, Tet was a tactical defeat but a strategic victory for them. I'm sure the planners were hoping for tactical victory, but it was the demonstration of the capability to strike so strongly so many places at once so unexpectedly--when the official U.S. consensus was that they were on the verge of being defeated--that made people in general in this nation give up on the idea that we could win that war: not bad for a 'Plan B'.
It can be argued that in the so-called Cold War of the U.S. vs. the U.S.S.R. Vietnam represented a tactical defeat but a strategtic victory of a kind for the U.S. After all, Vietnam was the last country to 'go communist' (though Nasser in Egypt, Assad in Syria, and Hussein in Iraq all expressed 'socialist' leanings--using the the Ba'ath Party to obtain power--and received material support from the U.S.S.R.). Demonstrating how far we were willing to go there assured the Bolsheviks that we would go even further in places that meant more to the U.S., such as Europe or the western hemisphere. So perhaps it at least made the leaders of the U.S.S.R. more cautious than they would otherwise have been--and avoiding a war with the U.S.S.R. while 'keeping communism contained', assuming our 'values' (or our economic power) would triumph in the end, was our penultimate goal.
One of the unexpected benefits of WWII for this nation followed from the democratic nature of that war. People from all kinds of different backgrounds were thrown together in a common effort to which all involved were comitted. People saw frst hand that we are all more alike than we are different, no matter what.
The war in Vietnam was nothing like that. The fact that it could never be like that was the best argument against getting militarily involved there in the first place. Even if it was a victory of a kind, it was way too dearly achieved and whether it was necessary will always be at the least open to debtate. It also set the precedent for military actions such as the one in Grenada, the two in Iraq, and the one in Afghanistan.