When using Add Include, Visual Assist inserts an include directive whose path delimiter, '/' or '\', is identical to the delimiter of existing #include directives in the file. If the current file does not contain any #include directives, or none of the existing directives have a path delimiter, Visual Assist uses a delimiter based on the current IDE.
Visual Studio 2010 and newer
See the specific IDE setting in documentation for Add Include.
Visual Studio 2008 and older
Visual Assist reads the default delimiter from the registry:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\DefaultAddIncludeDelimiter = "\"
Set this to either "\" or "/".