Do I need a visa if I am from New Zealand and my partner from Australia?

After searching online I have found so much conflicting information about this. We will be travelling to Vietnam next year on May 23. We would like to get Visa on arrival because we have been told that a Visa is only valid for 3 months from date of purchase. We will be travelling to South America on February 17th and will be backpacking so won't be able to send our passports to Vietnam Embassy. We will be in South America for 3.5 months, so of course no use applying for the Vietnam visa before we leave because it would expire before we got there. If we can get Visa on arrival to Vietnam (Hanoi), that would be much easier. We will be in Vietnam for 2 weeks from May 23. We fly into Hanoi and will depart overland from Ho Chi Minh City to Cambodia. Thankyou