I made a little research about furigana fields and discovered that in vCard they are stored in an extension with the values:

X-PHONETIC-FIRST-NAME:
X-PHONETIC-LAST-NAME:

Giving as a result a file like:

BEGIN:VCARD
VERSION:3.0
N;CHARSET=SHIFT_JIS:姓 名;;;;
FN;CHARSET=SHIFT_JIS:姓 名
SORT-STRING;CHARSET=SHIFT_JIS:ｾｲﾒｲ
X-PHONETIC-FIRST-NAME:
X-PHONETIC-LAST-NAME:
...
END:VCARD