Since you'll already have a visa for Vietnam, leaving the airport shouldn't be a problem. I was in Hanoi in May and I can tell you it wouldn't be the most interesting airport in which to spend six hours . The food is quite expensive (well, by Vietnamese standards, that is ...) and not the best quality. I really regret having my last bahn mi in Vietnam at this airport - hardly the greatest example of my favourite sandwich.
Unfortunately, there isn't much to do in the immediate vicinity of the airport and you're looking at around a 40-minute to one-hour taxi ride into the city - so if you consider this, as well as leaving a couple of hours to check in and get through security (which can be busy), you'll only really have around two hours to look around.
Be sure to get a reputable taxi company. Mai Linh is the company usually recommended and you'll find them outside the airport.
If I only had two hours to look around Hanoi, I would head straight to the Old Quarter and check out the interesting stores and markets in the area. Grab a bahn mi and a Vietnamese coffee and have a stroll around Hoan Kiem lake.
I'd also consider the time of day your stopover falls in - if you need to get back to the airport during rush-hour times, I wouldn't risk it. It might be a boring airport, but boredom is better than missing a flight!
"I travelled to Kuala Lumpur a few months ago in this seat - it does not lie completely flat. Almost.
"However I was extremely impressed by the seat, cabin and service.
"The extra stop at Gold Coast is not ideal, but it's very quick, you can have a meal on the first leg from Auckland, re-board at Gold Coast and sleep all the way to KL.
"If you are generally an Economy passenger, you will be delighted!
"Someone who is used to flying First/Business on a premium airline, say Air New Zealand, will not find the seat to be as good - but hopefully will find comfort in the price."