Try this:
$sql = "SELECT * FROM HTG_VSP WHERE HomeOfEmpNumber IS NULL OR HomeOfEmpNumber = '' OR HomeOfEmpNumber = %s or CustomForEmpNumber = %s";
$query_EmpVSP = sprintf($sql,GetSQLValueString($colname_EmpVSP, "text"),GetSQLValueString($colname_EmpVSP, "text"));