The concept of leaving a tip in Vietnam used to be alien to many restaurant service staff. However, given inflow of tourists and expatriates in Saigon and other cities, tipping is becoming widely known and accepted without drawing quizzical looks from the service crew.
To answer the question directly, no. You are not required to tip, but the servers and staff will always appreciate it. If you are not comfortable with the idea of tipping, then just consider rounding up your bill to the nearest ten-thousand and leaving the change behind.
For information on restaurants go to A Day in The Life of la Villa French Restaurant in Saigon