function GetCharFromVKey(vkey: Word): string;
var
  keystate: TKeyboardState;
  retcode: Integer;
begin
  Win32Check(GetKeyboardState(keystate));
  SetLength(Result, 2);
  retcode := ToAscii(vkey,
    MapVirtualKey(vkey, 0),
    keystate, @Result[1],
    0);
  case retcode of
    0: Result := '';
    1: SetLength(Result, 1);
    2: ;
  else
    Result := '';
  end;
end;

// Использование:

procedure TForm1.Edit1KeyDown
  (Sender: TObject; var Key: Word;
  Shift: TShiftState);
begin
  ShowMessage(GetCharFromVKey(Key));
end;