The 480 calories in bun thit nuong is fairly evenly spread, which is why people underestimate it. Nothing on the bowl looks heavy. The bun is 215 for 200g. The pork is thin sliced shoulder marinated in sugar, fish sauce and lemongrass, then grilled, so 80g is about 170 with the sugar glaze. The mo hanh spoon on top is 30 to 40 on its own, the peanuts are 15, and the sweet nuoc cham you pour over is around 40.
The marinade is the hidden bit. Vietnamese grilled pork gets a lot of sugar or honey in the marinade, which is what makes it caramelise on the charcoal. A quan may use 10g of sugar per portion of meat, another 40 calories that does not show up as fat. Combined with the sauce, one bowl can carry 20g of sugar.
The standard order in most Saigon quan is bun thit nuong cha gio, with one or two fried spring rolls chopped on top. Each cha gio is 130 to 140 calories, so the popular version of this bowl is really 620 to 750, not 480. Add the second ladle of sauce that comes when the bowl gets dry and you are at a full dinner's worth.