I do not think this is grid related, but due to the designed ViewBox behavior. The ViewBox tries to porportionally size and scale its children so the children cannot be set to autosize as this prevents the gridsplitter from working. If you set some explicit widths on the gridcolumns, then the sizing will start to work. But you also probably want to set the ViewBox to stretch in one direction to avoid the empty client areas if it tries to scale to fit in both directions. Here is the sample back.
http://www.syncfusion.com/support/user/uploads/SplitterViewBox_76f1f12c.zip