Also, what was the effect of this on the combat effectiveness in Vietnam?
I'd like to know about the extent it actually effected the units, both platoon and company level but also on regiment and division level. I'm aware that it reduced the combat effectiveness but I'm unware of the true extend.
Lastly, how common place was the use of drugs and alcohol in each year?
The part of your question about whether, during the Vietnam War, members of the Australian Army drank under-age isn't clear. It is effectively meaningless.
In Australia, drinking ages varied by State and Territory but the Australian Defence Act 1903 Section 123A overrides any State legislation and provides authorization to sell, supply, consume, purchase or have in possession any intoxicating liquor, to members of the Defence Force and their guests (not being school cadets) where authorized, there was no age limit. The authorization is delegated well down the chain of command to unit level.
In Vietnam, all of the EM clubs served, and all of the PXs sold, at least beer to anyone in uniform. In the PXs, the amount was limited by your ration card. I don't know about hard liquor, I never had a taste for the stuff. I have a vague memory that it was limited to E-5s and above.
Before 1969, drug use was rare; after 1969 drug use, chiefly marijuana, opium and heroin, was ubiquitous.