Working @ the moment
If you go to the UK Vodafone web site my account section and
click remind me
you can now enter a user name think first and last name or forum nick name etc and if they are a registered vodafone customer it will tell you their email address and if you click not the right number button it will then tell you their mobile number.
enter the Vodafone number and it will show the email address.
[Only registered and activated users can see links. ]
EDIT: NO LONGER WORKING