If you plan to go to Vietnam and then Cambodia and back to Vietnam, which I did this summer, you must get a multi-entry visa or you will have to pay for another visa like I did. This cost me an extra $195.00 for my wife, daughter, and me. You might ask why would you renenter Vietnam. If you are in Ho Chi Mihn City in the south, you will want to visit Siem Reap, Cambodia before going to Hanoi in the north. On arrival in Cambodia we had to pay $20.00 each for the visa and around $25.00 each for the exit tax when we left.
Have a great trip.