How to correctly pass attributes to the selectList field in VirtueMart?

Question

Joomla

I'm writing a small plug-in for VM, there was a gag with the selectList field - the passed parameters are not saved.
The code is standard, based on vmcustom:

function plgVmOnProductEdit($field, $product_id, &$row,&$retValue) {
  $helper = new plgVmCustomAttachFilesHelper;
  if ($field->custom_element != $this->_name) 
    return '';
  $file_filter = $helper::getFilter($field->custom_params);
  $list_files = array_combine($helper::listFiles($file_filter), $helper::listFiles($file_filter));
  $html ='
    <fieldset>
      <legend>' . vmText::_('VMCUSTOM_ATTACHFILES_ADD_FILES_LABEL') . '</legend>'.
      '<table class="admintable">'.
        VmHtml::row('selectList', 'VMCUSTOM_ATTACHFILES_SELECT_FILES_LABEL', 'filelist', 'customfield_params['.$row.'][filelist]', $list_files, 1, 'multiple="multiple"', '', 'VMCUSTOM_ATTACHFILES_SELECT_FILES_PLACEHOLDER').
      '</table>
    </fieldset>';
  $retValue .= $html;
  $row++;
  return true ;
}

From what we managed to understand from the documentation for the VM API and by trial, the parameters in the selectList are passed like this:

VmHtml::row(
  'selectList', //type
  'VMCUSTOM_ATTACHFILES_SELECT_FILES_LABEL', //label
  'filelist', //field name
  'customfield_params['.$row.'][filelist]', 
  $list_files, //input array
  '1', //size
  'multiple="multiple"', //milti
  'class="inputbox"', //attrib
  'VMCUSTOM_ATTACHFILES_SELECT_FILES_LABEL' //placeholder
)

two questions:
1. what is the error that the form does not save the parameters?
2. how to pass the default parameter?