my_save_vid_pix_data()

<?php
add_action( 'init', 'my_save_vid_pix_data' );
function my_save_vid_pix_data() {
    // ... other code here remain intact ...

    // Sorted post/attachment IDs.
    $vid_pix_sorted = [];

    foreach ( $atts as $vP => $arr ) {
        $vid_pix_sorted[] = $vP;
    }

    // Saves the sorted post IDs, in the exact order they were sorted on the
    // front-end.
    update_post_meta( $post_id, '_vid_pix_sorted', $vid_pix_sorted );
}