It seems that the Sort Selected Lines feature is not enabled for txt files, nor for other 'unknown' file types. It only seems to work for cpp and h files (we are working on c++ projects).
In our projects we have several other (plain text) files (.dep files, .mod files) that have special meaning in our build system. It would be nice to be able to use the Sort Selected Lines feature in those files, but is seems to be disabled.
Please try adding the custom txt file extensions to the following registry key. HKEY_CURRENT_USER\Software\Whole Tomato\Visual Assist X\<IDE spec>\ExtPlainText
Remember to shut down your IDE before editing the registry.