another way is to use standard VM search and use Joomla Site Seach statistics for that, in VM 1.1.2 and J 1.5.8 works following: add following code snippet to virtuemart_parser.php, somewhere line 180, see it with preceding line:
$keyword = substr( urldecode( vmGet( $_REQUEST, 'keyword' )), 0, 50 );
// log the search in JoomlaSearch statistics by turvas
require_once(JPATH_SITE.DS.'administrator'.DS.'components'.DS.'com_search'.DS.'helpers'.DS.'search.php' );
SearchHelper::logSearch( $keyword);