Vietnam has pretty large bills. Do to you inflation you have 500k bills. Sometimes is easy to get a 50k bill and think it's a 500k. The same can happen with 20k and 200k. So make sure you get familiar with the money by a search on the internet images of the bills before you travel to Vietnam. Most sellers are honest and will tell you if you pay to much for something but some sellers will take advantage of you if they notice you have no idea about the bill values.