It would be worth asking the question 'moving your number when taking out a new contract through a third party' on the forum search facility where you will be able to find some previous threads with examples on how to move the number.
As you will be able to see from previous threads, the ongoing SIM only contract will need to be cancelled by requesting Vodafone to put the number on PAYG or porting out to any other network and porting back in on the new third party contract. Either way, to keep the same number will leave you liable for all the early termination fees. If you keep both contract then you will be charged for 2 contracts.
A good way to avoid this is to take out the new phone upgrade contract direct with Vodafone, they will then write off what remains of the SIM only when taking a new agreement Evo contract with a phone. See Evo here: Evo
An additional benefit of going direct with Vodafone if you only pay the April CPI increase on the airtime part of the contract, with the third party you pay this i-in the entire amount. Having mentioned that, hold out until after the April CPI increase, it's likely to be a hefty one, quite sure I remember last year contracts taken out after the end of February didn't incur the increase.
