I'm of the understanding that iPhones, unless bought direct from Apple lock to the network of the first SIM inserted.
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.
As far as I know iPhones purchased via VM are NOT flagged to be locked on the apple servers. In fact vary few providers lock them now unless they are sold as part of a pre paid deal. (Pay as you Go).
I will also add a iPhone 4S i got from VM some time ago was not locked.
The myth is that only iPhones from apple is are not locked.