Add the following code to your Project.Build.cs:
PrivateDependencyModuleNames.AddRange(new string] { "VaRestPlugin" });
PrivateIncludePathModuleNames.AddRange(new string] { "VaRestPlugin" });
Then add the appropriate headers from VaRest like this:
#include "Classes/Json/VaRestJsonValue.h"
#include "Classes/Json/VaRestJsonObject.h"
You may also need to modify the UVaRestJsonValue class definition to look like this:
class VARESTPLUGIN_API UVaRestJsonValue : public UObject
instead of:
class UVaRestJsonValue : public UObject
I think that is all