class FormTestVerticalTabsForm

class FormTestVerticalTabsForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return '_form_test_vertical_tabs_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $tab_count = 3;
        $form['vertical_tabs'] = [
            '#type' => 'vertical_tabs',
            '#default_tab' => 'edit-tab' . $tab_count,
        ];
        for ($i = 1; $i <= $tab_count; $i++) {
            $form['tab' . $i] = [
                '#type' => 'fieldset',
                '#title' => t('Tab @num', [
                    '@num' => $i,
                ]),
                '#group' => 'vertical_tabs',
                '#access' => \Drupal::currentUser()->hasPermission('access vertical_tab_test tabs'),
            ];
            $form['tab' . $i]['field' . $i] = [
                '#title' => t('Field @num', [
                    '@num' => $i,
                ]),
                '#type' => 'textfield',
            ];
        }
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->cleanValues();
        // This won't have a proper JSON header, but Drupal doesn't check for that
        // anyway so this is fine until it's replaced with a JsonResponse.
        print Json::encode($form_state->getValues());
        exit;
    }

}