Problem while using the user_reference field in my bundle

Hi, I am creating an entity for my D7 Module and trying to include a user_reference auto-complete field in it. I am using the References module for this purpose. The autocomplete-field shows up correctly with the add-more option since I have set cardinality to unlimited.

However, nothing shows up in auto-complete suggestions and it doesn't accept any of the user-instances.

Even more annoying is that when I go and check the field settings in
Home » Administration » Structure » Content types » » Manage fields
and click on Save without making any changes, the suggestions start appearing correctly as they should.

I have written down the following settings for the field and the instance.

field :

'vocabulary_authors' => array(
      'field_name' => 'vocabulary_authors',
      'type'        => 'user_reference',
      'settings' => array(
        'referenceable_roles' => array('2','3'),  // for authenticated user + administrator
        'referenceable_status' => array('1'),    // for active
      ),
      'default_widget' => 'user_reference_autocomplete',
      'default_formatter' => 'user_reference_default',
      'cardinality' => FIELD_CARDINALITY_UNLIMITED,
    ),

instance :

'vocabulary_authors' => array(
      'field_name'  => 'vocabulary_authors',
      'label'       => $t(' Authors'),
      'bundle' => 'vocabulary',
      'entity_type' => 'node',
      'description' => '',
      'required'    => FALSE,
      'widget' => array(
        'type'    => 'user_reference_autocomplete',
        'settings' => array(
         'autocomplete_match' => 'starts_with',
        ),
      ),

Please help me out here. I think I am I missing out on some required settings.

Thank you.