attribute.php

add_filter( 'vc_gitem_template_attribute_post_custom_meta', 'vc_gitem_template_attribute_post_custom_meta', 10, 2 );

function vc_gitem_template_attribute_post_custom_meta( $value, $data ) {
   /**
    * @var null|Wp_Post $post ;
    * @var string $data ;
    */
   extract( array_merge( array(
      'post' => null,
      'data' => '',
   ), $data ) );
  $atts_extended = array();
  parse_str( $data, $atts_extended );
  
  $atts = $atts_extended['atts'];
  $output = print_r($atts, true); // will containt shortcode ATTS
  $output .= print_r($post->ID, true); // will contain current POST_ID
  $output .= print_r(get_post_meta($post->ID), true); // will contain current post meta..
  return $output;
}

init.php

add_filter( 'vc_grid_item_shortcodes', 'my_module_add_grid_shortcodes' );
function my_module_add_grid_shortcodes( $shortcodes ) {
   $shortcodes['vc_say_hello'] = array(
     'name' => __( 'Say Hello', 'my-text-domain' ),
     'base' => 'vc_say_hello',
     'category' => __( 'Content', 'my-text-domain' ),
     'description' => __( 'Just outputs Hello World', 'my-text-domain' ),
     'post_type' => Vc_Grid_Item_Editor::postType(),
  );
   return $shortcodes;
}
 
add_shortcode( 'vc_say_hello', 'vc_say_hello_render' );
 function vc_say_hello_render($atts){
  /** @var $atts array - shortcode attributes */
  $atts = vc_map_get_attributes( 'vc_say_hello', $atts );
  return '{{ post_custom_meta:' . http_build_query( array(
     'atts' => $atts,
  ) ) . '}}';
 }