You should be able to get that done by talking to Live Chat (Contact Us at the bottom of the page). Although Vodafone don't block overseas connections to Live Chat, some people have reported difficulty getting on to it outside the UK. If that happens to you, giveit a couple of minutes, but post back here if it's still not working.
If you've had the phone for more than a year, and are the original purchaser, there shouldn't be a charge for unlocking. If you aren't the original purchaser, you'll have to ask them to request the unlock.
