If Visual Assist finds a .git subdirectory in a parent directory of the current solution, Visual Assist assumes the current solution is one module of a set maintained by a single git repo. Hence, Open File in Solution (OFIS) searches all files under that parent directory when include-all-files-in-solution-directories is enabled in the context menu of the OFIS dialog.
Prevent Visual Assist from expanding the set of searched files, i.e. retaining the solution directory as the root of OFIS searches, by modifying the registry:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\UseGitRootForAugmentSolution = 00
Set to 01 to restore default behavior.