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When I go out to get some Banh Mi, I would order Banh Mi + something (thit nuong, nem nuong and such). If I order just Banh Mi, all I get is just the plain bread. I think it is due to the way the language works. When we talk to our fellow native speakers we tend to assume the other person knows what we are referring to and hence we just say key words.
Same as in English, when you ask your friend what they are going to eat for lunch, most of the time you will get a simple answer like burritos or burgers. It is when the person is at the store or restaurant that they would decide what kind of burritos (chicken or beef burritos) or burgers (cheese or hamburgers) that they want.
TL;DR: Banh Mi is a general name for a food like burritos or burgers, but language barrier causes misunderstandings.