I know this is an old thread but just been looking into it myself. My site had my normal joomla pages in one menu and all my virtuemart categories were created in swmenu pro and also weren't showing up in xmap. What I did was just add a new menu item into my main menu, type virtuemart, into the main menu and xmap picks it up. That link should go to show all your virtuemart categories. If you have the latest version of xmap it will have the virtuemart plugin built in. Takes a while to generate the xml sitemap. Now I need to go and update all my old websites which won't be indexing properly either. whoops!