You should note than Mai Van has the sign of Agribank (which is actually the largest bank in Vietnam by total assets), so it is kinda an official foreign currency exchange location authorized by the bank. There's also always a security officer outside Mai Van, so you will always feel safe.
Ha Tam has no bank's sign at all. So it's purely a gold shop. By law, it is illegal to conduct foreign currency exchange at a gold shop (unless it is also authorized by a bank like Agribank as in the case of Mai Van). So in theory, if you get caught doing foreign currency exchange at a gold shop, the money can be confiscated by the police. They won't jail you or fine you anything (as you're only the customer), but chances are, your money will get confiscated. They may return you the money, but it can get highly troublesome. People said that the only reason Ha Tam can survive is because they bribe the police, so police doesn't care. But you will never know. Some day, they may change the police and maybe Ha Tam does not bribe them good enough. Then troubles will come. It's just not worth the hassles.
In my opinion, it's always better to change at a place that has an official license from the bank like Mai Van, even though the rate may be a little higher.
Even if you will never get caught, I don't think it's a good idea to support such an illegal activity that Ha Tam is doing (doing business without license, and bribing police). Vietnamese usually don't care about the law anyway (i.e. you can do anything as long as you don't get caught), but I think we should care. That's a moral issue.
When exchanging money, it's always a good idea to see whether they have a bank's sign or not.
