Linux: Handset software is not created equal and carriers have different setups if I was you flash vfnz handset software from Sam mobile it's simple and I have never had an issue
Linux
Edit: Your issue might be the band scan used by Spark compared to VodafoneNZ
This thread suggests flashing vodafone firmware may not be straightforward or successful:
https://www.geekzone.co.nz/forums.asp?forumid=97&topicid=223318&page_no=3