Telecom New Zealand currently markets the Samsung W531 as a worldphone, capable of roaming to destinations around the world.  The W531 is both CDMA and GSM capable.  Telecom disables the ability to use any other SIM card then the one they provide, therefore preventing the use of a Vodafone NZ SIM card in it.

This document provides a guide on how to unlock the phone so that you can insert and use a GSM SIM card with it.

Switching Off Sim Lock on the Samsung W531 Cell Phone
http://tech.nztechie.com/2008/01/switching-off-sim-lock-on-samsung-w531.html