var s1 = "привет"; //"привет" в сыром utf- 8 
var Stream1 = WScript.CreateObject("ADODB.Stream");
Stream1.Type =  2 ; //text
Stream1.Mode =  3 ;
Stream1.Charset = "utf-8";
Stream1.Open();
Stream1.WriteText(s1);
Stream1.SaveToFile("out.txt", 2 );
Stream1.Close();

var s1 = "привет"; //"привет" в сыром utf- 8 
var Stream1 = WScript.CreateObject("ADODB.Stream");
Stream1.Type =  1 ; //binary
Stream1.Mode =  3 ;
Stream1.Open();
Stream1.Write(s1); //error
Stream1.SaveToFile("out.txt", 2 );
Stream1.Close();

var b = fso.OpenTextFile("out.txt", 2 , true, 0 ); //write
b.write(s1);
b.Close();