You can change theme by going to themes in the admin panel of vbdrupal(
http://yoururl.com/?q=admin/themes or
http://yoururl.com/admin/themes , depending on whether you have clean urls installed). vbDrupal comes with defaultangy, which pulls your default vbulletin style, but the developers have also bundled phptemplate themes. These are themes developed for normal drupal sites, but which also work pretty seamlessly with drupal.
They are somewhat easier to customise, if you want a different look.