This is usually a problem with xmap.
Quote from: jenkinhill on April 18, 2012, 12:58:42 PM
There are many reports here on the forum that having Xmap installed without the current VirtueMartplugin prevents the VM component from registering in the database, the anwer there is to add in the plugin and use Joomla 2.5's discovery feature. If you already have the Xmap VM plugin enabled, then disable it, save, and then re-enable it. The link returns
more here http://forum.virtuemart.net/index.php?topic=101120.msg336461 and there http://forum.virtuemart.net/index.php?topic=108471.msg363003#msg363003