single entry or multiple entry visas?

Hello, We're flying to Vietnam in November, travelling from Hanoi to Ho Chi Minh, and then flying from Ho Chi Minh to Siem Reap (Cambodia) On our flight back to UK, we're flying from Siem Reap to London Heathrow via Ho Chi Minh. Does it mean we need a re-entry visa? Or, since we only change flights, would single entry ok?

Thank you.