A small update here...
I found that the file containing the input code for categories is product.product_category_form.php, which is located in administrator/components/com_virtuemart/html.
So I read again the tutorial for the allvideos plugin, and as fas as I can understand, fotis evangelou just inputs the code in order to make the plugin seen by the code, just before printing the page. I did quite the same in product.product_category_form.php, but with no result. To be more accurate, what I did was to change this
//finally close the form:
$formObj->finishForm( $funcname, $modulename.'.product_category_list', $option );
?>
with this
// start - enable "content type" plugin parsing
global $_MAMBOTS;
$_MAMBOTS->loadBotGroup( 'content' );
$row->text = $template;
$results = $_MAMBOTS->trigger( 'onPrepareContent', array( &$row, &$params, $page ), true );
$template = $row->text;
// end - enable "content type" plugin parsing
//finally close the form:
$formObj->finishForm( $funcname, $modulename.'.product_category_list', $option );
?>
What I did, was to input the same code fotis gave us for allvideos plugin, in order to use the simple image gallery plugin. However, my category description in virtuemart still displays {gallery}blabla/blabla{/gallery}
Any suggestions here?