Below is the code to add user information to iPhone Contact.
As I said, I don't know anything about vCard, but this code posted by malinois in their answer here may be useful:
ABAddressBookRef addressBook = ABAddressBookCreate();
ABRecordRef person = ABPersonCreate();
NSString *phone = @"0123456789";
ABMutableMultiValueRef phoneNumberMultiValue = ABMultiValueCreateMutable(kABPersonPhoneProperty);
ABMultiValueAddValueAndLabel(phoneNumberMultiValue ,phone,kABPersonPhoneMobileLabel, NULL);
ABRecordSetValue(person, kABPersonFirstNameProperty, @"FirstName" , nil);
ABRecordSetValue(person, kABPersonLastNameProperty, @"LastName", nil);
ABRecordSetValue(person, kABPersonPhoneProperty, phoneNumberMultiValue, &anError);
ABAddressBookAddRecord(addressBook, person, nil);
ABRecordRef group = ABGroupCreate();
ABRecordSetValue(group, kABGroupNameProperty,@"My Group", &error);
ABGroupAddMember(group, person, &error);
ABAddressBookAddRecord(addressBook, group, &error);
ABAddressBookSave(addressBook, nil);
CFRelease(person);