After 4 wasted hours, it seems there is actually not an issue on the vBulletin side. But Outlook 2003 does not show the email contents using the same character set that is specified in the headers, even if that character set appears to be correctly selected in the encoding menu. The email only displays correctly if the character set is manually changed in the client and then changed back to the original setting. So it would appear that this is a bug in Outlook 2003.