TBF, some nationalities are a lot harder to differentiate than others. It's because they share very close genetic links (essentially the same race), just that they are separated by a line in the sand as different countries.
Chinese and Korean are very closely linked. I've personally been mistaken as a Korean many times at restaurants, brothels, RNTs. They just start speaking Korean to me when I meet them, it's really funny.
Vietnamese and Chinese are also very similar. I think of Vietnamese as part of the Chinese diaspora. It's the same race, until you speak to them you can't really tell if they're Chinese or Vietnamese.
Other nationalities that could easily be mistaken are the Indochinese ones to the west of Vietnam. Laos, Cambodia, Thailand, Burma, all are the same ethnic race and again very hard to tell the diff until they speak.