probastes asi ?
procedure TForm1.Button1Click(Sender: TObject); 
var  
  cmd, par, fil, dir: PChar; 
begin 
  cmd := 'open'; 
  fil := 'rasdial.exe'; 
  par := PChar(edtEntry.Text + ' ' + edtUser.Text + ' ' + edtPass.Text); 
  dir := 'C:'; 
  ShellExecute(Self.Handle, cmd, fil, par, dir, SW_SHOWMINNOACTIVE); 
end;