Argo,
Some of the first US armed forces people in Vietnam were Coast Guardsmen, deployed in cutters on the rivers. But, I doubt there were 1.1 million of them per your figures, above, and the difference you noted.
A friend is considered a Vietnam vet, and I don't believe he set foot in Vietnam-he was a B52 machinist, stationed in Tailand.
He left most of his hearing in south east Asia.

Best,
Ted