MidCan5 is not wrong with respect to National Guard service during the Vietnam era -- you joined the National Guard (if you could -- places were limited and it usually took "pull" to get in) -- to avoid going to Vietnam. Everyone took this for granted.
However, by the late 1960s -- certainly after Tet -- no one wanted to go to Vietnam. Or at least very very few saw it by that time as defending America against Communism. Few people -- especially 18-25 year olds -- had a sophisticated understanding of Vietnam, or why we were there (official explanations or otherwise). It just seemed to be this endless nasty war we were stuck in -- and probably would not win.
Our political leaders were deeply divided, the American people were ambiguous, our own generation's most articulate members had already turned vehemently against it, the news media's reports were increasingly pessimistic. (I recall seeing a Life magazine cover, after Tet, with smiling NLF soldiers holding captured M-79 grenade launchers, which we had just finished training with. This magazine was passed around the barracks and studied intently.)
We now know -- or at least there is a good case for saying -- that Tet was a tremenous defeat for the Communists, destroying the NLF's human infrastructure in South Vietnam. But at the time it looked like we had been surprised by a popular uprising.
So there was a general, confused but real, feeling that this was not a war in which our homeland was endangered, but something else. I recall only one man who was an enthusiastic anti-Communist warrior, and he was generally a rather ridiculous person whom no one took seriously. (Nor did he end up in the infantry.)
The sergeants and officers were professionals and just did their duty without saying much. There seemed to be a fair amount of cynicism towards our political leaders, which as it turned out would be amply justified. But I suppose that's been true since Nebuchadnezzar's times.
What they tended to say to us was, "When you're over there it will be them or you, and they've been fighting for 25 years and have weeded out all the duds, so you better listen up." I was actually surprised at the lack of ideological content in the training.
I Basic I met plenty of people who, facing the draft, signed up as RA's because they were promised training that would get them out of the infantry. No one was ashamed of doing this. Everyone just discussed it from the point of view of whether the recruiters could be trusted on this issue, and whether the extra year a Regular had to serve was worth it.
Most people who were drafted, went. But it would be wrong to think either that (1) they only did so because they feared going to prison, and were thus a bunch of angry and rebellious conscripts, or (2) they just grimly did their patriotic duty. It was more complicated than that, although both of these descriptions would enter into a full description of the popular mood. (Things changed for the worse after 1968, at least in the lower ranks of the Army.)
There were plenty of kids who were not unhappy about being in the military, too. A number of them went airborne, which was definitely not the way to avoid combat. (I still recall our platoon sergeant at the end of Basic calling for "All you crazy people who want to jump out of airplanes, get over here", and a group of young men separating themselves out. Many were Latinos, by the way, which I always remember whenever I read some demented anti-Mexican ravings.) What mix of patriotism, the desire for excitement and a guaranteed proof of manhood, and the desire to be among an elite, made up their psyche's, I cannot say.
At least this is how it was among my training platoons in Basic and A.I.T. at Tigerland in the autumn of 1967 and winter of 1968.
I think this is why the American people have decided to give a pass to those who did not volunteer to go to Vietnam as Eleven-bravos, whatever their method of avoiding service was: getting a 4F, going into the Guard, fleeing to Canada, staying in graduate school, or pulling strings in some other way. The more you played by the rules, the more of a pass you get, but basically, everyone skates.
Don't blame the young people of those times for the decisions they made. They found themselves in a new situation, which did not fit the simplistic your-country-is-in-danger model they had grown up with. Many of the institutions of their society gave them mixed messages -- at best -- on what their duty was. "If the trumpet giveth forth an uncertain sound, who shall prepare himself for the battle?"
If you want to blame anyone, blame the polticians who threw several million young Americans into a war that they thought could be won by graduated responses and half-measures, against an absolutely determined, ruthless and experienced enemy who had limitless support from Russia and China, and a safe hinterland to retreat to and launch attacks from.
The next time I see a soldier I will bow to the ground and say: "Your Highness and Holiness, I will worship at your feet. Please dispense thy special wisdom and rights unto one such as I. I know your shit isn't the same color as mine." 
