1. Here is information from the Vietnamese Customs Department:
Narcotic medication drugs must be declared to customs and must not exceed the quantity stated in the enclosed doctor's prescription, but not more than the quantity indicated for seven days.
Psychotropic medication drugs or medication containing precursors used as drugs must be declared to customs and must not exceed the quantity stated in the enclosed doctor's prescription, but not more than the quantity indicated for ten days.
Medicines must have labels clearly stating the name of the drug, name of active ingredients, concentration, content, expiry date and must have a prescription from a physician. A prescription must contain at least the following information: patient's name and age; drug name, strength or concentration and volume; the amount of medicine (or the number of days of taking it); dosage; full name and signature of the physician; the physician's address (hospital, clinic, centre…).
2. In common understanding, addictive/narcotic medications are referred to opioid/cannabis and similar products. Psychotropic medication is any drug capable of affecting the mind, emotions and behaviour. For your reference (please click on the link): Narcotic medication drugs, Psychotropic medication drugs, precursors used as drugs. The lists may be wider with variants and similar products. Please consult your doctor for details
3. If you need to use more than 7 days with narcotic medication drugs, 10 days with psychotropic medication drugs or medication containing precursors used as drugs, you can make a declaration at the Viet Nam's customs of the port of entry where you will enter with your itinerary, proof of neccesity (from doctors). The customs will decide to let you bring in or not. You can contact a hospital in Viet Nam before your travel to know if they can supply medications for you.