I guess you are talking intellisense and not build? I marked you post as (intellisense)

It's being redefined either by vs or the toolchain. It is possible we need to add an #undef before we re-define it.

Please provide the info requested in yellow above so that we can see what you have installed or try this...

Create a vs-intellisense-fix.h in the __vm folder next to the vsarduino.h.

#undef __cpluplus
#define __cplusplus 201103L
 

If the #undef works let us know we will add to the core.

I created vs-intellisense-fix.h
- was not recognized by VS
- I added it manually to the project: No effect.

I downloaded the vsix from them link. Could not be installed. This is the log:

16.06.2020 07:05:36 - Microsoft VSIX Installer
16.06.2020 07:05:36 - -------------------------------------------
16.06.2020 07:05:36 - Initializing Install...
16.06.2020 07:05:36 - Extension Details...
16.06.2020 07:05:36 -      Identifier         : c2b94433-1774-4242-8062-987a4484f1ed
16.06.2020 07:05:36 -      Name               : Arduino IDE for Visual Studio (Visual Micro)
16.06.2020 07:05:36 -      Author             : Visual Micro
16.06.2020 07:05:36 -      Version            : 2020.518.8
16.06.2020 07:05:36 -      Description        : A fully compatible Arduino Edit, Build , Deploy and Debug tool. All Arduino IDE versions are supported. This is an admin install for all users. (optional Arduino debugger and more!). Supports Arduino and all clones (AVR, SAMD, ESP, STM32, Intel, Ti etc.). Supports Hadrware/GDB and Serial debugging of mulitple .ino, cpp files, libraries and core. Teams can split code into shared projects.

NOTE: This extension is upated frequently, please switch off auto-update via 'tools>extensions and updates' if the update prompt annoys you. 

Latest release provides a number of fixes and additions. Please see the 'Release Notes' at www.visualmicro.com

Visual Micro supports many different types of hardware and code libraries. These can be installed manually or using the automated installation tools provided by this extension.
16.06.2020 07:05:36 -      Locale             : en-US
16.06.2020 07:05:36 -      MoreInfoURL        : https://www.visualmicro.com/
16.06.2020 07:05:36 -      InstalledByMSI     : False
16.06.2020 07:05:36 -      SupportedFrameworkVersionRange : [0.0,2147483647.2147483647]
16.06.2020 07:05:36 - 
16.06.2020 07:05:36 -      SignatureState     : Unsigned
16.06.2020 07:05:36 -      Supported Products : 
16.06.2020 07:05:36 -            Microsoft.VisualStudio.Pro
16.06.2020 07:05:36 -                  Version : [15.0.27220,16.99)
16.06.2020 07:05:36 -            Microsoft.VisualStudio.Community
16.06.2020 07:05:36 -                  Version : [15.0.27220,16.99)
16.06.2020 07:05:36 -            Microsoft.VisualStudio.Enterprise
16.06.2020 07:05:36 -                  Version : [15.0.27220,16.99)
16.06.2020 07:05:36 - 
16.06.2020 07:05:36 -      References         : 
16.06.2020 07:05:36 - Signature Details...
16.06.2020 07:05:36 -      Extension is not signed.
16.06.2020 07:05:36 - 
16.06.2020 07:05:36 - Searching for applicable products...
16.06.2020 07:05:37 - Found installed product - Global Location
16.06.2020 07:05:37 - Found installed product - AtmelStudio
16.06.2020 07:05:37 - VSIXInstaller.NoApplicableSKUsException: This extension is not installable on any currently installed products.
   bei VSIXInstaller.App.InitializeInstall(Boolean isRepairSupported)
   bei VSIXInstaller.App.InitializeInstall()
   bei System.Threading.Tasks.Task.InnerInvoke()
   bei System.Threading.Tasks.Task.Execute()