I guess the sandwich is “ubiquitous enough in everyday conversation” that it received its own entry.
As for the definition of bánh mì in the OED? It is “a Vietnamese snack consisting of a baguette (traditionally baked with both rice and wheat flour) filled with a variety of ingredients, typically including meat, pickled vegetables, and chili.”
In addition to bánh mì, phở has also its own entry in the OED. I’m feeling hungry now!