You cannot sort within QViewer, but you can sort using EasyMorph.
From QViewer, open the QVD in EasyMorph using either the "Morph It" button in File View or the "Morph It" right-click in Folder View.
In EasyMorph, click the dropdown for the desired column and select "Sort".
Note that if you use the EasyMorph Export action to write the sorted contents back into the QVD file, EasyMorph will not preserve fields Comments, Tags and NumberFormats. Those attributes will be lost.