That thought had crossed my mind. Not knowing how or where you aquired the piece, especially if you did not get it yourself "in country", if you picked it up from a surplus shop, garage sale, etc. with out any story behind it, that thing could have come from any where at any time.
A guess the real question is why do you want to know. If it is just because you are curious, then by all means, continue the search for knowledge. If it is because you are doing research and you need to know what kind of holster to give a Vietnam U.S. figure then I would say use it at your own risk. As previously stated, it is not of any known issue, but, it is possible that it could have been aquired by some other means. It could have been made/purchased in Vietnam. I have seen many examples of locally made holsters. It could have been made by the user himself or someone in his or another unit. I have also seen examples of this as well. But I think the main thing to remember is that pistols were not a standard issue to everyone. You only get one issued if your an officer or your job dictates one. If that is the case then you usually get the holster issued with it and in this case it would be, most likely, brown or black leather, stamped U.S.
Does that mean that one could not be aquired by other means? Not at all, hence the need to come up with a way to carry it.
I have no idea why but subjects like this get the best of me. It is those little obscure things that I love.