foreach($virtuemart_category_id as $catId){
$childcats = $catmodel->getChildCategoryList(1, $catId, null, null, false);
foreach($childcats as $k=>$childcat){
if(!empty($childcat->virtuemart_category_id)){
$cats .= $childcat->virtuemart_category_id .',';
}
}
$cats .= $catId; //here missing comma, should be $catId .',';
}