Can I add an attachement field that holds a vCard?

You can use a Java lib for this like https://github.com/mangstadt/ez-vcard or you just create a vcard conform String and write it into a File. For information how a vcard looks like see the wiki page http://en.wikipedia.org/wiki/VCard

You would need to map the fields to your code, then give them a button to run the scrip and save the new file.