If you think about it, money exchange is just essential side business for jewellery/gold shops - what do you to when a customer shows up with bunch of their own currency? And gold is, well, gold standard, pun intended, carrying universal exchange rate built-in.
These shops are at/nerby every big/medium market/bazaar in Vietnam. They can be easily identified because all of them have glass walls and blight of gold jewellery is coming from there.

This is not a joke. It is not intended to to freak you/your wife out (however here in this forum there were cases when people decided against travel after reading some stories) but just to prepare and having right expectations. Just follow common sense and advices people are giving in forums and you will be fine.