This is an OK topic guess, as it illustrates the transitional nature of military medicine. During Vietnam, as during WWII, many changes in equipment happened gradually. My experience was in US Navy, and with the USMC and I recall we were using plastic by 1969-70. Of course syringes are used in many settings and for different purposes. In outpatient care for injections of vaccines and medications, plastic was routine by 1970. However in a surgical setting, or for procedures, glass was still used well past the mid 1970's and beyond the Vietnam era. Insulin syringes were plastic by the late 1960's. As far as glass goes, by Vietnam most but not all, were of the interchangeable type (as opposed to individually fitted barrels and plungers). Also most syringes in this era had a locking tip like the Luer-Lok, but not all did. Many had a plain tip just like the WWII types. For needles, we still used all metal re-useable needles but by about 1969 they were all disposable. All needles had metal hubs however. I mostly remember using glass syringes in surgery and for certain procudures like putting in CVP lines. They were much nicer to use too.
So, what was actually used, and what was in the store room could vary a great deal. Hope this helps.