As John noted it was Agent Orange that finally got Andy. It's one hell of a note to die from a war 30-40 years after it's over. And every year it gets a few more of us.

OK. So let's talk a bit about those things that were unique(to one extent or another) to Vietnam and made that war an especially tough one to fight.

Leadership:
Larry has already mentioned "shake&bake" NCO's. An NCO that leads men in combat NEEDS to have had combat experience and have experience with, knowledge of, and the respect of, the men he's leading into combat. There just really isn't any substitute for these things.
The above is also true for officers and the situation may even have been worse where they are concerned because they were constantly being moved into and out of leadership positions. The result(for those who survived) was that just about the time they became a competent leader-and the unit began acting like the team they were supposed to be-they were put behind a desk somewhere and the unit was stuck with some green guy fresh from OCS who didn't know a bush from a bayonet.

Instead of allowing the military to do it's job there was a very great deal of micro-management in Vietnam by politicians who had (apparently) never even HEARD OF a bush or a bayonet.
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Individual replacement:
This has to have been the worst idea was ever come up with by the paper-pushers. I think it was supposed to be more "cost effective"(might have been as far as money goes; but not lives).
In the the traditional way of doing things a unit trains together, becomes a team, and then goes to war together. When they were withdrawn they went home as a unit. Many of us were sent to Vietnam as individuals and then plugged into whatever unit needed warm bodies. When your time was up you went home by yourself. leaving the guys whose backs you were watching to make do as best they could. Did you dare try to find out later if they were OK? What if they were dead because you were no longer there doing your job? Talk about survivors' guilt! When our units were "brought home" those with any appreciable time left in their tours went instead to other units needing warm bodies.
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Pressure:
It's no news to anybody that wars tend to be somewhat stressful. This is true of any war. But in most wars a soldier is able to believe that he has the support and best wishes of the folks he is told that he's fighting for. Many of us had very good reason to believe that the average American citizen hated our guts, and that the government and military could care less about our welfare as long as we did what we were told. If not they actively hated our guts too. Enemies in front, enemies behind, enemies all around. Any wonder we formed such strong bonds? We were all we had. Then we (some of us) came home. We weren't heroes but we had a right to be proud of who we were and what we had done. We had stood up and put our lives on the line doin what we were directed to do just the same as those men who created this country and have routinely paid the price to keep it free. But nobody cared or wanted to know. Most of us got the message pretty quickly that we would be tolerated only so long as we kept quiet. Not good.