The bánh mì as we know it was born in the 1950s, after the French were defeated at the Battle of Dien Bien Phu. Left with many French staple dishes (but finally free to modify according to their local ingredients), Vietnamese people living in Saigon are said to have created the sandwich as the antithesis to the popular French baguette sandwich. In the bánh mì, vegetables replaced the expensive cold cuts, and mayonnaise was used in place of butter.
The bánh mì only found its way to America after the Fall of Saigon in 1975, when millions of Vietnamese people fled their home country for new ones — like the United States. There, many immigrants opened restaurants to feed their fellow Vietnamese communities, which, many years later, resulted in the mainstream American popularity of this sandwich.
Source: https://www.buzzfeed.com/rossyoder/national-dishes-from-different-countries-cuisines