Bánh Mì Xíu Mại is the meatball reference point for the whole family, the version every other one is measured against. The name carries its lineage: xíu mại is the Vietnamese reading of the Cantonese siu mai, and the Chinese-Vietnamese kitchens of the south turned the steamed dumpling filling into a loose, soft pork meatball simmered in a slightly sweet tomato sauce. Dropped into a rice-flour baguette with the usual đồ chua, cucumber, cilantro and chilli, it becomes a sandwich that eats almost like a soup you can hold, the bread doing the work a spoon would otherwise do.
The meatball itself is the thing to understand before any of the variants make sense. Good xíu mại is ground pork with a deliberate amount of fat, bound lightly with a little cornstarch or soaked bread and seasoned with shallot, garlic, fish sauce, pepper and a touch of sugar. It is not packed tight like an Italian polpetta; it is mixed gently so it stays tender and almost crumbles under the bread's pressure. Some cooks add minced water chestnut or jicama for a faint crunch inside the softness. The sauce is built separately from fresh tomato cooked down with shallot, a measured spoon of sugar and fish sauce, and often a little of the pork fat for body, then the meatballs poach in it until they have traded flavour with the gravy in both directions. The classic failure modes are easy to taste: a dense, bouncy meatball means it was over-worked or over-starched, and a thin, ketchup-sweet sauce means the tomato was rushed and the sugar overdone. The bread matters as much as the bowl, because a soft loaf turns to paste under all that liquid; the baguette has to be fresh enough to keep a brittle shell while the inside soaks.
Service tells you which tradition you are eating. In one common style the meatballs and sauce arrive in a small bowl alongside the bread, and you tear, dip and rebuild bite by bite, which keeps the crust crisp to the last piece. In the to-go style the filling is spooned straight into the split loaf with đồ chua, herbs and chilli, and you eat it before the bread surrenders. Both are correct; they solve the same sauce-versus-crust problem from opposite ends.
The branches off this trunk are well populated and worth treating separately rather than blurring together here. There is the meat-free build with mock meatballs for those eating chay; the version layered with pâté for a second register of richness; and the one named for its sauce, where the tomato gravy is the headline rather than the meat. Each shifts the balance enough that it deserves its own article rather than being crowded in here.