If you want to stock your kitchen with the bare minimum for preparing Vietnamese food, start with good quality fish sauce (nuoc mam). How do you judge fish sauce? Look for a light amber color and the words nhi or thuong hang on the label.
These terms indicate that the condiment came from the first extraction of liquid from the fermented fish, and is of the highest quality. Grades of fish sauces are similar to that of olive oils. That is, extra virgin olive oil is more flavorful and costlier than virgin olive oil. This same rule applies to judging different fish sauces.
Also keep in mind that fish sauce is also used in Thai and Filipino cooking, where it tends to be saltier and heavier in flavor. So, even though Thailand produces most of the fish sauce sold in the U.S., you need to make sure that the condiment you’re buying is made in the ‘Vietnamese’ style. How to do this? Look for Vietnamese lettering alongside the Thai script.