ajax delete value from custom field array

Try this:

function delete_attachment() {
    $id = $_POST['att_ID']; // the attachment ID

    // Deletes the attachment (and all of its custom fields or meta data; e.g.
    // `photo_order`). The `true` bypasses the trash.
    $att = wp_delete_attachment( $id, true );

    // Deletes all custom fields named `vid_pix` where the value (`meta_value`)
    // is the attachment ID (i.e. `$id`).
    if ( $att ) {
        global $wpdb;
        $wpdb->query( $wpdb->prepare(
            "DELETE FROM {$wpdb->postmeta} WHERE meta_key = 'vid_pix' AND meta_value = %s",
            $id
        ) );
    }

    die();
}