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Good adviсe. However, I can't call myself an inexperienced foreigner since I spent several years in Southeast Asia altogether.

Overcharging can happen anywhere and any time in Vietnam. That's why I told this above

...people aren't spoiled by tourism. (But it doesn't mean you should completely relax: know real prices, ask how much you should pay before getting food or buying anything else.)

Food hygiene is more a question of common sense. Vietnamese street food in general is seriously behind Thailand from the point of hygiene.

Alas, many travelers prefer positive thinking to common sense.