It’s possible to enter Cambodia or Laos from Vietnam and then re-enter without having to apply for another Vietnam visa. However, you must apply for a re-entry Vietnam visa before you leave Vietnam. If you do not have a re-entry Vietnam visa, you will have to go through the whole Vietnam visa process again.
Re-entry Vietnam visa is easiest to arrange in Hanoi or HCMC, but you will almost certainly have to ask a travel agent to do the paperwork for you. Travel agents charge about US$25 for this service and can complete the procedure in a day or two.