Add the "override" identifier when using Implement Interface / Virtual Methods via the options dialog for Visual Assist.
Visual Assist build 2036 and older
Add the "override" identifier in C++ commands by editing the following registry BINARY value:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\UseOverrideKeywordInImplementInterface = 01
Set the value to 00 to return to the default behavior.