Pre-requisites

• Joomla 2.5.x
• Virtuemart 2.x
• FTP Client (Filezilla)
• PHP Editor (Notepad++)

As a first step, you need to enable this functionality which can be found in Virtuemart's core configurations:

1. Login to your Joomla! Backend
2. Go to Components > Virtuemart > Configuration
3. On the Shop Page, tick the option that says "Enable Joomla Plugin"
4. Save & Close.

By doing this, you are now able to call and parse custom Joomla plugins into Virtuemart's product description.

The next step is hard coding this functionality to the category description:

1. FTP to your server (by using Filezilla or any other desktop/mobile client)
2. Go to your /public_html//components/com_virtuemart/views/category/view.html.php and download it (Don't forget to create a backup copy of the file, since you're going to add your custom code to it).
3. Open view.html.php in your favorite PHP editor (Notepad++, Dreamweaver or any other desktop/mobile client)
4. Insert the following code snippet on line 151:

// add content plugin //
$dispatcher =& JDispatcher::getInstance();
JPluginHelper::importPlugin('content');
$category->text = $category->category_description;
jimport( 'joomla.html.parameter' );
$params = new JParameter();

if(JVM_VERSION === 2 ) {
	$results = $dispatcher->trigger('onContentPrepare', array('com_virtuemart.category', &$category, &$params, 0));
	// More events for 3rd party content plugins
	// This do not disturb actual plugins, because we don't modify $product->text
	$res = $dispatcher->trigger('onContentAfterTitle', array('com_virtuemart.category', &$category, &$params, 0));
	$category->event->afterDisplayTitle = trim(implode("\n", $res));
	$res = $dispatcher->trigger('onContentBeforeDisplay', array('com_virtuemart.category', &$category, &$params, 0));
	$category->event->beforeDisplayContent = trim(implode("\n", $res));
	$res = $dispatcher->trigger('onContentAfterDisplay', array('com_virtuemart.category', &$category, &$params, 0));
	$category->event->afterDisplayContent = trim(implode("\n", $res));

} else {
	$results = $dispatcher->trigger('onPrepareContent', array(& $category, & $params, 0));
}

$category->category_description = $category->text;

Save the file and re-upload view.html.php to the same destination and voila! you're done.

You can test your newly added code by going to your website's Joomla Backend > Components > Virtuemart > Products > Product Categories, and create / edit a category and attach to it any plugin related call (e.g. { gallery }...{/ gallery } for some of Image Gallery Plugins).