You will usually be served tea and something to eat. Generally this will be Vietnamese green tea or soft drinks. Be sure to sample them. Failure to taste or drink a small amount of anything is considered impolite. Have your written materials translated into Vietnamese.
Many Vietnamese today have or are studying English or other foreign languages.
Most, however, still feel more comfortable with their own language. Most foreigners will also need to use translators. If there is any question about the comprehension of English by the people you are meeting, always use your own translator or interpreter. Do not rely on the other organizations translator or interpreter. They work for the other side and are more likely to protect their business interest and to focus on their interests not yours.