To avoid insane crowds at the grocery stores in Vietnam, go shopping during lunchtime on a hot and sunny day.
Also, if the shithead with a smug-but-guilty look on his face cuts in line in front of you, don’t give him the satisfaction of getting away with it unchecked. Call him out in English in front of everybody and stare him down until he pays and leaves.
Even though no one will understand your repeated muttering of “Not cool, dude”, they’ll all understand your shaking head and the clearly perturbed look on your face. The nice thing about shitheads is that they’re universally recognized.