Creating vCards online is necessary if you do not have an eMail- or Addressbook-Client, since most eMail-Webservices do not support exporting single contacts.
Here is an overview of relevant services for creating contact-files online
For more information on how to operate these services take a look at Christina Shaffer's article here.
P.S. If you have the contact-details embedded as a signature in an eMail, it might be efficient to use vicarda.com