I finally figured out, direct relationship to distance from the docks.
Saigon = Budweiser, as much as you want.
Quin Nhon = Miller, plenty of it.
Pleiku = Schlitz, usually.
Kontum = Schlitz/Falstaff, if the convoy arrived on time.
Dak To = PBR or rusty Carling. Falstaff if a big ship unloaded recently and if the beer truck made it through the ambush. Whatever else you can steal. Otherwise, "Potable Water."
Any Place outside the base camp wire = two dented, rusty cans of whatever swill nobody else will drink, and you better like it and drink it fast because if you don't somebody else will. And you're out of water.