Not at all, at least not from what I read a few years ago about this. The war situation there was frankly unfathomable to us privileged folk. Beside the obvious physical pain and injuries there were psychological influences which are normally so far removed from our lives that we cannot deal with them. Drugs are an escape from the physical injuries and pain, and then they turn out to be an escape from the mental awareness.

Shooting at other humans, killing them, is not something we are designed psychologically to handle. But obviously if you feel you must kill another to avoid being killed, you may do it. And then your mind must reconcile that memory. Drugs can help you avoid it.

The change of attitude has absolutely nothing to do with laws.

Your partially stated assumption here is that soldiers stopped using drugs because of the punishments. I think this is a case of post hoc fallacy. Yes, punishments create a disincentive for some behavior, but only in the case of rational actors who have the means to act on that motive. Some soldiers who left Vietnam had the necessary support systems to overcome addiction or were never addicted in the first place or weren't in an environment where those drugs were available at home. Others did not and stayed addicted, even when they came home. Heavy penalties don't necessarily cause a proportionally smaller addiction problem. They just punish heavier. The only rational path to reducing drug addiction is to improve the conditions that cause drug addictions. Very few people become drug addicts for no reason.