Were you signed up to a top up offer before you choose the offer with the international minutes? If not, you would be charged roaming fees, which would be expensive. Also check your apn settings and see if it's set to live.vodafone.com. Make sure that it is. If it's not, it'll use up your credit.
If that doesn't work, contact live chat or send a PM to one of the mods with the following and they can look into it for you:
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Apologies for the delay in responding. If you send us a private message (via the link in my signature) with the details in the above post we can take a look into this for you?
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