Visa exemption and re entry

Hello, me and my friend are british passport holders, we are travelling to Vietnam, into Cambodia, and then returning to Vietnam for 1 night to return home to the UK. Our time in Vietnam before going to Cambodia is under 15 days, when we re enter Vietnam would we need to purchase a visa on arrival? And if that is the case, is it better to apply for a visa online before we first enter the country and be able to use it to re enter Vietnam the second time?