I thank your Dad for his service but he must not have seen what my numerous Vietnam buddies did. 
At least a dozen of them I know well learned to smoke in Nam and continued after returning stateside. And these guys were not rear echelon troops but those who were sent deep into enemy territory very often (I've seen their medals). Rangers, LURPs, Loch pilots, medics, grunts and officers; they smoked on patrol or when flying helicopters and they lived to tell their tales. Many believed it enhanced their perception in the jungle which is a positive thing. It was not always recreational. The Army must have known how widespread this was. No stereotype just a different group of men than your Dad served with with a different perspective..