Tap water is not safe in this country. Bottled water will always be the best option. You also need to be careful about ice in drinks; factory-made ice is generally safe, but be wary of anything else, especially in rural areas.
Tropical diseases such as malaria, dengue or Japanese encephalitis are endemic in rural Vietnam. Malaria is not a huge concern in the bigger cities such as Ho Chi Minh and Hanoi, but ensure that you bring your mosquito liquid repellent with you. Thanks to much improved hygiene conditions in recent years, cooked food sold by street vendors and in restaurants, including blended ice drinks, are mostly safe. Just pay attention to the sources of the food you eat, and you will be fine.
The quality and availability of healthcare is particularly poor in rural areas, and in a few parts of the country it is impossible to access healthcare. Medical personnel working at public hospitals will generally only speak Vietnamese. However, private hospitals in Vietnam are generally excellent. These hospitals tend to address the needs of expats and accept international health insurances.