I tried to import a vCard file with ISO-8859-1 encoding. Preferred encoding
for outgoing vCards was set to Unicode (UTF-8).

But Outlook imports the vCard file with UTF-8 encoding, resulting in wrong
non ASCII characters like German Umlauts.

The vCard looked like this:

BEGIN:VCARD
VERSION:2.1
N;CHARSET=ISO-8859-1:äöüß;öäüß;;
....
END:VCARD

If the charset value is written in lower letters, Outlook detect the
ISO-8859-1 encoding.

BEGIN:VCARD
VERSION:2.1
N;CHARSET=iso-8859-1:äöüß;öäüß;;
....
END:VCARD

But RFC1521 says: Unlike some other parameter values, the values of the
charset parameter are NOT case _sensitive_.

Who knows why Outlook acts like that?