How to enter text in a fieldset's summary?
See vertical_tabs_example_form_alter(), there it's written:


  Vertical tabs provide its most usable appearance when they are used to
  include a summary of the information contained in the fieldset. To do
  this, we attach additional JavaScript to handle changing the summary
  when form settings are changed.


  '#attached' =&gt; array(
    'js' =&gt; array(
      'vertical-tabs' =&gt; drupal_get_path('module', 'vertical_tabs_example') . '/vertical_tabs_example.js',
    ),
  ),


In vertical_tabs_example.js it then goes on like so:


  Use the fieldset class to identify the vertical tab element. Depending
  on the checkbox status, the settings will be customized, so update the
  summary with the custom setting textfield string or a use a default
  string.


$('fieldset#edit-vertical-tabs-example', context).drupalSetSummary(function (context) {
  if ($('#edit-vertical-tabs-example-enabled', context).attr('checked')) {
    return Drupal.checkPlain($('#edit-vertical-tabs-example-custom-setting', context).val());
  }
  else {
    return Drupal.t('Using default');
  }
});