There are some products which you may find for sale here which you should avoid. These may be illegal in Vietnam or your home country, or simply may encourage destruction of the environment and harm to the local people. These include:
Coral and coral products. Buying these products encourages destruction to Vietnam's irreplaceable coral reefs.
Sea Turtle products. You may find preserved sea turtles in shops - particularly sold in the open in Hanoi. This should be obvious--but sea turtles are endangered and all products made from them are illegal.
Rice wine containing whole animals or animal products. Wildlife populations have been decimated in Vietnam. Many of the animals and animal parts used as ingredients in rice wine are globally threatened and endangered - including the snakes.
Ivory, bone and tooth products. This animal product will be illegal in most home countries, regardless of which animal it came from. The only allowable products may be those made from farm animals--but this may be very difficult to distinguish.
Alligator and crocodile leather. This is illegal to transport between many countries.
Drugs - including marijuana are illegal and the sale or use of them can carry SEVERE penalties.
Prostitution is illegal and destroys not only yourself, the person you engage with, and your family. Aids and other serious STD's are VERY COMMON in Vietnam--even the countryside.