Can't speak for other units, but the stories I heard from friends at the time suggest it was pretty widespread.
The unit I was in was awash in heroin in `70-`71.Heard
2 shots when some bro`s shot a major trying to arrest
them smoking mj.
Yeah,you could get those little vials of scag for about
$2 and put it in your Marlboro and smoke it like normal,
Then there were your hard core that warmed it up.
A wee problem with race relations,yeah.It was scary where
I was.I liked it better out in the field,
I saw first hand only up until mid-68 and only in the 1st Inf Div....the news references about it didn’t begin until long after that.......I’m sure it was available, but it wasn’t popular, so to speak, until later..........geez, when I got drafted in fall of 66 marijuana in the states wasn’t very widely and openly available yet except for places like NYC, SFO, etc. I am absolutely certain I knew more guys in Vn who smoked their first reefer THERE than had ever tried or even had a chance to consider junk. But I do also know that what you say is and was true 2 yrs after my DEROS. By that time the “Woodstock generation” was arriving in Vn.