Ask VF for a copy of the agreed contract, Get them to point out the section that allows price increases.
Until they supply it, ring them or post a ,message here that you don't agree with the change in price and you don't agree to it until they can provide the agreed contract which details allowing price increases.
Vodafone has quoted the portion of the contract that allows price increases.
Basically they can do anything they want - if it's advantageous to the customer they don't have to give notice - if it's disadvantageous to the customer they have to give 30 days notice and the customer has the right to terminate the contract without hving to pay the break fee.
"1.3 We can change any of our terms and any of our Plans, Services, or Charges at any time. We can also withdraw any Plan or Service at any time. We can move you to a comparable Plan or Service if we withdraw your Plan or Service, or we think you would be better off. We may notify you in advance depending on the nature of the change:
a) If we consider that the change will have a neutral or positive effect on you, we can make the change without giving you notice;
b) If we consider that the change will have a detrimental effect on you:
we will give you 30 days’ notice of the change or, where that is not possible, as much notice as possible;
subject to (iii), if you cancel or transfer your plan, we will waive any early termination charges or transfer fees;
iii. if you cancel or transfer your plan where there is an increase to Charges due to a tax or levy imposed by law or a charge from a third party supplier of a direct input required to provide the Service (including a charge for a regulated input required to provide the Service), we will only waive any early termination charges or transfer fees where the change will have a material detrimental effect on you."
While what they are doing is allowed by the contract, it certainly isn't within the spirit of the meaning of having a fixed term, fixed price agreement.