I know with previous versions you could buy an unlocked iPhone instore with no contract.
The website seems to insist on using a payment plan.
Can you buy an unlocked iPhone 6s outright in a vodafone store?
Don't see why not.
I'm sure you could. Otherwise, just buy it straight from Apple: http://www.apple.com/nz/shop/buy-iphone/iphone6s
Why not just order it from Apple?
Navigate there via the "browse all mobiles" pull down, rather than the "Iphone" one
then you can get to this, with the "mobile only" option