hassle and bargain
Overcharging has long been an issue in Vietnam tourism. It can happen anywhere on anything from an hotel room, a ride on taxi, coffee, meal, clothing, basic grocery stuff. Your coffee suddenly becomes 100% more expensive and a restaurant may present you an English menu with inflated prices. A friendly local who spent 30 minutes talking with you may also feel like overcharging you on anything.
Vietnamese hold a diverse view on this issue but in general it is more common in Vietnam than other neighbouring countries to see it socially acceptable to overcharge foreigners. They may argue inflated prices are still cheap and they may blame on the cheap cost of living which attracts a lot of backpackers with barebone budgets. According to this school of thoughts, if tourists complain about it, it's because they're stingy. Rich tourists from developed countries should not have a problem being overcharged.
The good news is that standard price is much more common than early 90s. You will absolutely spoil your travel if you assume that everyone is cheating you, just try to be smart. In a restaurant, learn some common dish names in Vietnamese, insist that you need to read Vietnamese menu, and compare it. If owners argue that the portion of dishes in the English menu is different, it's definitely a scam and move to other places. Learn some Vietnamese numbers and try to see how much a local pays a vendor. Also try basic bargaining tactics: Think how much it is back home, ask for big discount and walk away, pretending that the price isn't right. Many products tend to be standardized and compare more.
Try to be as clear as possible on the agreed price. You may agree 20.000 VND with a "Xe Om" driver for a specific trip, but at the end he may claim you are due 40.000 VND. Then you pay 20.000, smile and say goodbye, because you have a good memory. Once I agreed USD 1 for shoe cleaning, but afterwards he said USD 2 for two shoes, and more for "badly needed" repairs for my pretty new shoes. Ony after tour guide intervention I came away with 50.000 VND USD 2,50. If you agree price for a tour in a "cyclo" perhaps simply because you are tired of saying "no", they are likely to show you something additional, and ask more money than you initally agreed. On the Danang-Hoian bus, I found that even prices on public buses have to be negotiated oddly enough typically if you are already under way - but they are unliky to kick you off the bus if you are not prepared to pay enough in their perception.