I flew Asiana ten years ago on a purchased C class ticket. One of the flights was in the first class cabin. I if it's the same thing now but only two classes were offered. The F cabin was used for J passengers while the J cabin was used for economy (I guess they sat their best customers there). Flights to Vietnam tend to have a lot of demand for economy and less for premium traffic, so I guess that's how they do it. The service, lounge access and points you get might only be based on business class.