Have you tried emailing [email protected]? If you scroll down to the very bottom of their website, you will see that they list that email address in the footer.

https://www.vueling.com/en

Added: Both craigthemif and LondonElite are absolutely correct in stating that airlines and other travel companies generally refuse to make changes to reservations via email. So, be certain to note that you do not have the ability to call them.