Vietnam offers a wide range of cuisinary choices, you may decide to have your meals in a luxury restaurant/hotel or in a sprawling market, sometime in a food vendors when roaming around winding streets. But to avoid the uneccessary hygiene accidents, please follow some below tips:

In the restaurants/hotels:
Wash your hands because many Vietnamese dishes are used with bare hands
Order to have wet towel before enjoy your meal

In the market:
Pick the clean and trusted food shops
Avoid to order the dishes cooked by grease
Clean your chopsticks and bowls before using

In the street:
Street food is a not-to-be-miss attraction in Vietnam but carefully choose the stalls with many customers due to the quality and hygiene of food are often appreciated by number of customers.