In vB, IP addresses of visitor messages are stored in long format (I know, it's kinda random considering that elsewhere vB uses the dotted string presentation for IPs). In order to correctly import IP addresses to xF, the following line has to be fixed in the vBulletin.php importer module:
...since subsequently logIp() which is used expects a dotted IP address string.
'ip' => $vm['ipaddress']
'ip' => long2ip($vm['ipaddress'])