Health issues (and the quality of medical facilities) vary enormously depending on where you are in Vietnam. The major cities are generally not high risk and have good facilities, though rural areas are another matter.
Travellers tend to worry about contracting infectious diseases in Vietnam, but serious illnesses are rare. Accidental injury (especially traffic accidents) account for most life-threatening problems. That said, a bout of sickness is a relatively common thing. The following advice is a general guide only.