The vCard standards generally require certain characters to be escaped when used as part of a field value. These characters include commas (\,), newlines (\n or \N), backslashes (\\), and semicolons (\;). Claws Mail doesn't seem to be aware of this rule; when displaying field values, the escape character is displayed literally. For example, take the following vCard: BEGIN:VCARD VERSION:3.0 CLASS:PUBLIC EMAIL:webmaster@example.com FN:Webmaster\, Example Industries N:Webmaster;;Example Industries;; UID:EjqLRGm4gL END:VCARD This gets displayed in the Claws Mail address book and in the drop-down address completion as "Webmaster\, Example Industries". It should actually be displayed as "Webmaster, Example Industries".
Only version 2.1 vCard is suported in Claws Mail, which uses quoted-printable to escape such characters. Version 3.0 (like yours) or greater are not supported yet, so I'm turning this into an enhancement request.