Replace Visual Composer / WPBakery Page Builder default image when no featured image present

replace_vc_gitem_post_image_background.php

function replace_vc_gitem_post_image_background($output){
    // Get the URL to replace in string background-image: url('https://yoursite.com/wp-content/plugins/js_composer/assets/vc/vc_gitem_image.png') !important;
    $url = get_site_url(null, '/wp-content/plugins/js_composer/assets/vc/vc_gitem_image.png');
    // Full http(s) path to image
    $new_url = 'full-uri-to-new-image.ext'; // 1024x1024 image
    // String Replace
    $new_output = str_replace($url, $new_url, $output);
    // Return new string
    return $new_output;
}

add_filter('vc_gitem_template_attribute_post_image_background_image_css_value', 'replace_vc_gitem_post_image_background', 10);

Awesome, thanks!