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;