This falls under supplies of goods or services, which is covered by SoGA, and depending on how you purchased your dongle and credit distance selling regulations may also apply.
You should know by now because I have frequently reminded forum readers, that in every retail situation it is a duty of the customer to ensure the goods or services they are purchasing are fit for purpose. This includes how any changes to the service is managed. Communication services are rather intangible, so they tend to be surrounded with lots of terms and conditions. It is assumed that when you agree to take on such a service you have checked the T&C's and find them 'fit for purpose'. Ideally you should have the opportunity to read AND understand the T&C's offered by the company before you sign up, but in recognition of the reality that T&C's are often complex and need reading carefully, (something which is not readily done in a shop) some contracts of this nature have a statutory 'Cooling Off' period which gives you the right to cancel without penalty within that period. If you do not cancel or challenge the T&C's within the cooling off period you are deemed to have fully accepted the T&C's which then form a binding element of the contract.
Once the cooling off period has expired there is no realistic comeback against the company, provided the all the provisions of the T&C's have been adhered to by all parties.
Before you waste time and money trying to recover your unsued credit from Vodafone, I urge you to read the terms and conditions very carefully. I'll bet they include in the small print how the company can change or withdraw the service, but it should also include how they will inform customers of any impending changes.
It should also specify how the company will handle unused credits, or recover outstanding debt's.
If the T&C's do not cover all those bases, then you may have a chance of having the T&C's or contracts deemed unfair by the courts. But if it does cover them then sadly you don't have a leg to legal stand on, as you agreed to the T&C's. The only possible way may be to show them the moral card, but unless there is a mass protest by lots of affected customers I don't hold out much chance of you getting your money back. It will I am sure be cheaper to put it down to experience.
I don't agree that companies should be able to hold onto unused credits where the facility to use them has been withdrawn, It is tantamount to theft, and I think the recent OFGEM instruction to energy companies to try and repay the credits they hold, is a step in the right direction. So I suggest you lobby OFCOM and your MP to make it statutory for service providers to make every effort to refund all unused credits.