Yes,I would say more often than not they do ask.But it wasn't the first time that they didn't. I know a Hongkong Chinese with a British passport who also flies back and forth ,Saigon ,Bangkok, always vietjet on visa exemptions,who told me that they rarely ask him. Could be because he speaks perfect Vietnamese. I think the risk for airlines is minimal. Probably doesn't happen that often that they have to fly someone back, and in the end that someone will be made to reimburse the airline.