“Trời ơi “ (“ơi dzời ơi “ being the northern version of “Oh My God”) is the obvious go-to phrase. It comes up in the top ten words that foreigners learn when coming to Vietnam, and is now unoriginal for veterans. But “trời đất ơi “ is the real hidden gem because it doesn’t just mean “Oh Sky”, it modifies it with “Oh Sky and Earth”, an obvious reference to Taoism. The religious references don’t end there. There’s the Christian reference - “Chứa ơi “ (“Oh Christ”) and the Buddhist - “Mô Phật “ (“Homage to Buddha”) and it all crystallises in “Hết Hồn “ (literally meaning “No more soul”), which you’d use if someone surprised the bejesus out of you. Finally, to round off the religious tones there’s “Quỷ Sứ “ (“Devil”), which you’d call someone jokingly, or non, if they did something inappropriate.
There’s some Confucian impolite stuff out there too. There’s the obvious “má mày” (“your mother”) that we see the world over, but it gets really Confucian with slangs like “Gì vậy Ông Ngoài?” (“What’s the deal (maternal) grandpa?”), which actually is not restricted to being said to actual grandpas, any male will do. You could also go with “Ông Nội “ ((paternal) “grandpa”) in the same situation. And finally, “Mất Dạy “, which literally means “Lose (your) teaching”, can be one of the most insulting things a person can say to another because it basically means your parents didn’t raise you right.
But just as the title of this article suggests, you shouldn’t actually use any of these insults or expletives. Maybe in shared circles with friends you’re sure you can call “mày” (an impolite way of saying “dude”) and they won’t get offended, but for Confucius’s sakes, let’s all get along.