The .hson library has always had it's way of doing things

I don't think visual studio c++ intellisense likes the use of "namespace".

Microsoft are making good headway moving towards better support for the gcc/c++ system. We will try to make use of that during the coming months which will help.

In the meantime you might find atmel stdio and visual micro a better solution

or do something lame a make intellisense dummies for the json functions. 

In Visual Studio the _MSC_VER constant is only defined for intellisense purposes, not for arduino compile, so you can put your own headers in a .h along with #pragma once

In your sketch you can include your own local header instead of the real library, the compiler will use the real library but your intellisense will use your dummies.

#ifdef _MSC_VER
	#include <ArduinoJson_IntellisenseOnly.h>
#else
	#include <ArduinoJson.h>
#endif
 

This might not be practical with arduinoJson because of its complexity.