Remember that rumor about “Mama” Cass Elliot from the Mamas and Papas? The one about how she died in bed while munching on a sandwich? I heard that as a child and for years I took it as a cautionary tale about bedtime snacking, or else as evidence that fate has a rather whimsical sense of humor. It would serve a third purpose equally well: as a warning for those about to indulge in Vietnamese banh mi — the hulking behemoths of the sandwich world.