Can I upgrade to VM2.0.18a from VM2.0.12 or any older version of VM2.0.? Is it right way to upgrade?
Yes, use the Joomla extensions installer for the update of both the main and the aio components, existing data will not be lost (although you should always try the update on a backup copy first).
See the live update notes for anything that may affect your current site: http://virtuemart.net/index.php?option=com_content&id=416