You can make Visual Assist recognize non-standard extensions via a tweak to the registry, as described is this Knowledge Base article:
http://support.wholetomato.com/default.asp?W328
Or follow these instructions:
For VS2013, add your extension to ExtSource in the following location in your registry. Remember to include the dot and terminating semicolon in the value:
HKEY_CURRENT_USER \ Software \ Whole Tomato \ Visual Assist X \ VANet12 \
Replace VANet12 with the devstudio version for other IDEs you have--you should see multiple entries in the Visual Assist X branch if you do.
Press Rebuild on the Performance tab of the Visual Assist options dialog and restart your IDE(s).
Assuming you have Visual Studio 2010 or newer, add your extension to:
IDE | Tools | Options | Text Editor | File Extension | Microsoft Visual C++
IDE | Tools | Options | Projects and Solutions | VC++ Project Settings | Extensions To Include
Build 1929 should suffice.