You'll find the solution here: http://gist.github.com/572781
Let me copy it here for convenience and future reference:
class ProjectConfiguration extends sfProjectConfiguration
{
public function setup()
{
$this->setupProjectPlugins();
}
/**
* Responsible for initiating any plugins and pointing vendor plugins
* to the "vendor" subdirectory
*/
protected function setupProjectPlugins()
{
$this->enableAllPluginsExcept(array('sfPropelPlugin'));
$vendorPlugins = array(
'ioMenuPlugin',
'sfCKEditorPlugin',
'vjCommentPlugin',
'sfThemePlugin',
'sfDoctrineSlotPlugin',
'sfFormExtraPlugin',
'sfFeed2Plugin',
'sfImageTransformPlugin',
'sfDoctrineGuardPlugin',
'isicsBreadcrumbsPlugin',
'sfDoctrineActAsTaggablePlugin',
'sfInlineObjectPlugin',
'ioEditableContentPlugin',
);
foreach ($vendorPlugins as $plugin)
{
$this->setPluginPath($plugin, sfConfig::get('sf_plugins_dir').'/vendor/'.$plugin);
}
$this->enablePlugins($vendorPlugins);
}
}