Several features of Visual Assist, e.g. Code Inspection and Open File in Solution, apply special treatment to 3rd-party source. By default, Visual Assist determines that a file should be designated 3rd-party if its directory matches the regex:
You can change the regex by modifying the following string value in the registry:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\ThirdPartyRegex
Delete the registry entry to restore its default value.