Update entry with a reference number including $entry['id']

<?php
// more complete version of updating an entry with the entry ID as part of a reference number
// after the form was submitted
// change the 19 here to your form ID
add_action("gform_after_submission_19", "set_reference_number", 10, 2);
function set_reference_number($entry, $form){

    global $wpdb;
    // which field do you want to add the reference number to?  Change 18 here to your field ID
    $field_number = 18;
    // this is the format for the reference number to store
    $reference = 'VSR-' . $entry['id'];     // change this to whatever you want to store
    $wpdb->insert("{$wpdb->prefix}rg_lead_detail", array(
        'value' => $reference,
        'field_number' => $field_number,
        'lead_id' => $entry['id'],
        'form_id' => $entry['form_id']
    ));

}