Did you mean to title the thread, 'VS2015 include header from VS2013'?
Normally, I would ask you to try clicking 'Rebuild' and 'Clear' in the following menu. (VAssistX | Visual Assist Options | Performance)
Then, restart your IDE.
However, since the issue still remains, try typing out the correct include like '#include <future>'. Then, delete the include and try to add the include through Add #include Directive. This could help ensure Visual Assist knows about the correct file.