OK, I would say that this is a bug. I tested this under 22.214.171.124.0 and also got 'null' as result. It doesn't matter if the vo is based on a vo or not.
However, as long as you don't log a bug with support and it's not confirmed by support it not a bug.
As a workaround you can use the
String sql = vo.getQuery();
to get the whole sql statement. Then you can use normal string methods to extract the 'order by' clause.