Here is the disclaimer: I am not an economist and all my experience has come from living in Hanoi's Old Quarter.
It is up to the consumer to know what is a fair price for an item. Conversely, pricing in the United State by and large for most products is based on wholesale purchase price, as well as supply and demand. I have no idea how they determine pricing here. I do know that a price of an item is based on your ethnicity, and some assumptions about what you will pay, and how much of an asshole you will be in the negotiation process. But the baseline price for the average person. What is it and how is it determined? I have no idea. I doubt anyone knows.
I hate this bullshit negotiating process. They will charge me more than double what the Vietnamese pay. To me, that seems unethical. I am fine with paying an extra 10% or 1,000VND but double or more is ridiculous: Unethical. The whole process leaves me bitter. So, when I can I will buy at the local convenience store where things have a price listed. I thought that this problem was because I am a foreigner but it turns out that they do the same thing to each other. Well, at least it isn't personal or based on some sort of hatred. On the other hand, it makes me think this behavior is even more unethical. My Vietnamese teacher also hates this process and prefers not to do it but we all have to. I think whole situation is really paving the way for big business. Given the opportunity I would gladly pay more at a supermarket with market prices than to go through the hassle of negotiating with these mean people. And I gather the Vietnamese feel the same way. So, if the Vietnamese government were to ever allow big business markets to come in here it would clean up. First, all the small farmers would go out of business because big business would come in find or create large suppliers of perishable products. Then they will establish a huge supermarket with marked and non-negotiable prices, a one-stop shop. That would be the end of the fresh market vendors.