I'm using VA in different visual studio projects. In one project "Add include" inserts backslashes to separate the header file from the path. In other projects slashes are inserted. I'm not able reproduce under which conditions backslashes are inserted.
As our software is also supposed to compile with linux the usage of backslashes in include paths is simply a no-go.
VA uses the separator already used in the current file, it will also look for and respect the IDE setting on which separator to use. This FAQ entry explains how to tell VA to always use slash or back slash as the separator: