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Philippine Airlines are offering return flights to Hanoi, Vietnam for $659 return. This isn't the cheapest ever flight to Hanoi ($580) however this includes a 16 hour layover in Manila for free. It's actually a really well designed itenerary as flight leaves Auckland at 11:30PM and arrives 6:30AM in the morning so sleep on the plane, get up and then check out Manila until the outbound flight to Hanoi at 10:25PM.
Fares include bags, meals and seats. Dates are August to November and then February to March 2020. Philippine Airlines website has a search to find the cheapest dates to fly. Go there then plug the dates into Skyscanner, Momondo etc.
Sep/Oct is a good time to visit Hanoi — not too hot or cold, not too rainy.
You can get a Vietnamese travel visa on arrival: bring $30 US cash (yeah, seriously) and prepare for about a 1hr wait at the airport.
Take a day or two to visit Halong Bay (5hr bus from Hanoi), it's awesome.