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.