Visual Studio controls the installation of all packages which are installed via "Tools>Get Tools". Visual Micro does not have any control over that and does not tamper with the installation. If the following do not help then run a repair via "windows > add or remove programs > visual studio > repair"

The following might help.

1) Ensure your visual studio is up to date using Help>Check for updates". Microsoft have mad a number of changes to support more modern c++ tools but sometimes if modern packages are installed and the visual studio version is not up to date those changes break things.

2) Visual Micro uses the Linux/Embedded Tools c++ package. Not the C++ desktop package. Therefore, if c++ desktop is not working then ensure it is installed via tools>get tools

3) if you are testing standard c++ projects make sure you do not add a .ino of the same name otherwise that won't be a test of generic visual studio desktop c++ projects.

4) if you are trying to test an arduino project use "File>New>arduino project" or "file>open>arduino project" for the simplest test.

Are you sure the toolchain you have selected, such as desktop c++, have the #includes of <string> or <iostream> instead of <string.h> or <iostream.h>?

Thanks for the detail and the update.

As the compiler also rejects the #include <string> in both vMicro and Arduino IDE, it seems best to use the #include <string.h> file, which is found in the toolchain.

In desktop C++ projects these headers come from the VC++ toolchain installed by Visual Studio, however when using Visual Micro we have to use the toolchain included with each board package.

It is likely that some boards support both variants of the headers (e.g. ESP32 allows <string>), whereas the ATMega2560 only seems to support the *.h file, as it is the only one included.