It's a riveting history saturated with colonialism and migration. For a long time, sandwiches were a largely unattainable luxury in Vietnam due to high prices for imported wheat, and what sandwiches there were tended to look a lot more like French food. Jambon beurre, pâté, that sort of thing. In the 1950s, when France divided Vietnam, the Le family moved from the Hanoi area to what was then Saigon and opened a shop that basically reinvented and democratized sandwiches. These banh mi eventually proliferated throughout the world, and some are still made by members of the Le family. You can learn more about this fascinating history here.