I don't think that's a good comparison at all. From everything I've seen, you can get some outstanding food from a random cart in Vietnam for very cheap (probably like 1/3-1/4 the price a BK meal at this point), and it's going to be 1000x better than anything you could ever find in an American fast food place.
That said, the US also has incredible food, including street food and food carts in many places, that also blows fast food out of the water.
If you come here, and only eat fast food, I honestly feel really bad for you. Especially since it's not even cheap anymore.
The amazing food (and variety of it), is one of the biggest, most visible benefits of the US' multiculturalism. Anyone who has lived in a US city knows exactly what I'm talking about.