Insurance is a must for Vietnam, as the cost of major medical treatment is prohibitive. Although you may have medical insurance in your own country, it is probably not valid while you are in Vietnam.
A travel insurance policy to cover theft, loss and medical problems is the best bet. There is a wide variety of policies available, so check the small print. Some insurance policies specifically exclude such “dangerous activities” as riding motorbikes, diving and even trekking. Check that the policy covers an emergency evacuation in the event of serious injury.