Vietnamese stocks slumped for a second day in three as investors continue to take a breather after last week's sharp rally. Since jumping 5.2% in the first four days of last week, the index has mostly traded sideways, just below the 660 level. On Tuesday, having spent the session trading in a fairly narrow range of 662-657, the market closed down 1.36 points, or 0.2%, to 658.11.