The origin is much older than the 1950s, though it's difficult to pin down specifics. But surely the average Vietnamese was eating baguette decades before that. Already by 1880s, 'bánh mì' is appearing in Vietnamese-French dictionaries, a corruption of the French 'pain de mie'. And it was found even in the provinces, not just in cities. It's not clear to me, and is much more difficult, to discern when specific ingredients were associated with bánh mì (from butter, to eggs, to pate, etc). What is clear though is that they were widely eaten by at least the 1920s. Walking through Saigon, you would have seen Vietnamese relaxing in the morning drinking coffee and eating some form of bánh mì.

You can read a Vietnamese studies listserver discussion among historians & anthropologists discussing it here.