In most cities in Vietnam, there are many taxis brand names available, which are cars painted either in red or yellow or green. Taxi fares may vary from city to city but they are supposed to be marked on the taxi window, and the taxi drivers are supposed to use taximeter when you get on the taxi.
Most taxi drivers do not understand English, it is better for you to have your destinations written down in Chinese so that you can show the address to the taxi driver, and don't forget to ask for a receipt or remember the number of the taxi in case you leave something in the taxi.