To some people, haggling can be the most intimidating task. For some others, it can be annoying and time consuming. However negative feeling this activity causes for you, try to look on its bright side: after all, haggling is a whole cultural experience.
If you come from Europe or North America, chances are that most prices are fixed and well noted. Viet Nam, like many other developing countries, likes to play it hidden. Except for supermarket and restaurants, every transaction is open to be bargained. Imagine the feeling of triumph after you manage to push down the price of an item and you will feel encouraged to haggle.
The rule of thumb is that you need to know the price of certain things before you haggle. A polite beginning can be “Can you give some discount?” or “It’s too expensive for me”. If the shop owner does not show any sign that the price is fixed, you can continue your “battle”. Try two third of what was offered (or even half if you feel necessary) and haggle up until both sides reach an agreement.
Remember that the closer you are to the touristy area, and the richer you look, the more the product is overcharged. Soon you will find out it matters more the satisfaction feeling than reaching the actual value of what you want to buy.