Have your food cooked thoroughly. This is equal to refusing to eat any raw or half-cooked food, especially street food.
Drink only bottled water or chilled boiled water. Tap water is not drinkable in Vietnam so your water bottle will not be of much use here. You can buy a 1.5l mineral water and carry with you around. Try to avoid ice at all times.
Wash hands frequently, before eating and after using the toilettes. It might be best to keep a hand sanitizer handy and use it often as you can. Hand sanitizer is also available for purchase at most supermarkets in Viet Nam.
Avoid lettuce, unpeeled fruits and vegetables in street vendors. Local authorities also advise that you thoroughly wash fruits carefully before consuming.
Stay active. Travelling to an exotic country may sometimes mean overstuffing yourself with seafood and foreign cuisines. Help your digestive system by walking lightly after a heavy meals and stay active during the day with mildly rigorous activities such as hiking, trekking and swimming.