In fact, Vietnam was a pretty easy place to get heroin. It's manufactured very nearby.
Thailand, along with Myanmar (formerly Burma) and Laos, form the Golden Triangle-- one of the most prominent opium producing regions in the world. They held the title as the world's largest opium producer until Afghanistan overtook this infamous "honour" in the early 21st century. Vietnam is a neighbouring nation, and even in wartime-- perhaps especially in wartime-- the drug trade is a good business to be in, profit-wise. Heroin is one end-product of opium.
In the 19th century, the opium trade in Asia was enormous. 1838 featured the First Opium War between Britain and China. I'm not an expert on this war, but it was a fascinating moment in the history of the interaction between the two empires, and featured China viceroy Lin Zexu writing to Queen Victoria about her "moral responsibility to stop the flow of opium" into China. Opium was so common in China that it prompted many of the 19th century "moral panics" against allowing immigration by Chinese people into the USA and the UK-- they feared the rise of "opium dens."
Anyway, back to Vietnam.
What we might call "casual" use of heroin by American soldiers in the Vietnam War was not uncommon. This is an academic study about heroin use upon returning home to the USA (but the data is from 1970-1971 interviews, what an interesting blending of fields!) The American soldiers tended to be smoking or snorting heroin instead of injecting, which would be easier in their deployment context.
Only a minority of those who used heroin in Vietnam had ever injected it there. Most smoked or snorted it. After their return, most of the men who had used heroin in Vietnam used it very occasionally or not at all. Among those who did use heroin more than five times after their return, most had tried injecting it. Significant predictors of post-Vietnam injection included living on the east or west coast, deviant behavior both before and in Vietnam, opiate use before Vietnam, serious involvement with heroin in Vietnam, use of non-opiate drugs before and in Vietnam, and associating with drug users before Service.
Heroin is an interesting drug to discuss, though. Presently, it holds the stigma of being the Worst Drug Ever (tm)-- not unreasonably given both its addictiveness and its harm. However, heroin has not always been associated with the sort of Requiem for a Dream, needle in arm, endless despair narrative. In the 1990s in the USA/UK we had "heroin chic":
At the time during which heroin chic emerged, the popular image of heroin was changing for several reasons. The price of heroin had decreased, and its purity had increased dramatically. In the 1980s, the AIDS epidemic had made injecting heroin with unclean needles increasingly risky. Available heroin had become more pure, and snorting became a more common mode of heroin use. These changes decreased the stigma surrounding the drug, allowing heroin to find a new market among the middle-class and the wealthy, in contrast to its previous base of the poor and marginalized.
Heroin chic was associated with the rise of a sub-class of opioid use, the super-potent synthetic China White (which we would now call by its common name, fentanyl.) We can't talk about modern events, but obviously the current opioid crisis is not entirely without historical precedent of widespread use of opioids.
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