Providing you are the original ownwer of the phone you can complete this unlock request form:
https://www.vodafone.co.uk/vodafone-uk/forms/unlock-code-request/index.htm
Once Apple receive the unlock request from Vodafone they process the unlock and this can take up to 21 days. The unlock process is finalised by you connecting the phone to iTunes using a non Vodafone sim card in it.
Apple do not advise you when the unlock has been processed so it is a matter of you following the above steps day by day until you see the unlocked confirmation in iTunes.
