Banh canh is heavier than it looks because of the noodle itself. Banh canh strands are tapioca and rice, thick like udon, and they hold water without absorbing broth, so a 180g portion is 265 calories, about 20 percent more than the same weight of bun. The broth is the other surprise: it is thickened with tapioca starch until it coats the noodles, and 250ml of it is around 120 calories. Together that is 385 before any topping.
The crab is the good news. Real crab meat is 100 calories per 100g, so the 40g you get in a normal bowl is only 40 calories and mostly protein. The claw is decorative, maybe 15 calories of meat inside. The fish cake slice is 55 for 30g, and two quail eggs are 35. So a basic bowl is 380 and a full one with everything is 520; the published figures span exactly that range.
What pushes it past 600 is the extras that Saigon quan pile on. Cha (pork paste), a piece of gio heo, extra crab at 20,000 dong a scoop, and a tablespoon of fried shallots that is 40 calories on its own. A bowl dac biet with a crab claw, two kinds of cha and hock is 640 and it comes with a bread roll to mop up the broth, another 190.