I have no 3g coverage with Voda here in Whangarei as confirmed on their coverage map even though I am about 600m from their cell :-( but 2Degrees shows full coverage on their map. I thought that 2Degrees used Vodafones network outside of the main centres?
I am on 2Degrees. From what I understand they have their own network and cell towers. If your phone shows that you are on 2Degrees network then you are connected to one of their cell towers and therefore are using their network.
If you are roaming off 2Degrees network onto Vodafone's, your phone will say that it is connected to VodaNZ.