VSTO is functionality available in Visual Studio, which allows the user to craft a .NET Office Add-in to be shipped with an office document.
Usecase: Initial Access to the system
Create a VSTO document (Word, Powerpoint, etc) embedded with a DLL file written in .NET and send it to any user. Once the user opens the documents and approves the "Add-In" installation, code will be executed.
Utilize a different VSTO payload "Add-In" and include the code for any of the persistence techniques. After successful installation, persistence will be achieved. For more information, see "VSTO-POC" in references.