Rice paper is dried starch, so it is dense before anything touches it. The 60g of banh trang strips in a bag is about 200 calories, the same as a plate of bun. Then the seller adds what makes it taste good: a spoon of sate (chili in oil) and a spoon of mo hanh (shallot oil), together 12g of oil and about 90 calories, plus 10g of crushed peanuts at 60. The quail eggs are 45 for 3, the bo kho (beef jerky) and dried shrimp about 45. Mango, rau ram and kumquat are 10. Total 450.
It does not register as a meal because it is a bag you eat with a toothpick standing up. But 450 calories is a bowl of pho bo, and the Vinmec note that 94.5 percent of the energy comes from starch and fat means it does almost nothing for hunger. Most people eat one at 4pm and then have a full dinner at 7. Add the second bag that a lot of students buy and you are at 900 before dinner.
The size of the bag is the whole story. Vinmec's 300 per 100g is fine, but a cart outside a school in District 1 sells 100g bags at 15,000 dong (300 calories) and 200g bags at 25,000 (600). Banh trang cuon, the rolled version with the same toppings, is 350 to 400 for a similar weight because it uses less oil. Banh trang nuong, the grilled sheet with egg and cheese that tourists call Vietnamese pizza, is 250 to 300 per sheet and often two are ordered.