What is VSTA?
VSTA provides ISVs a customization technology allowing them to create more secure, manageable, and reliable applications taking advantage of Windows Vista, the 2007 Office System, and additional Microsoft development tools.
VSTA is a .NET-based application customization technology designed to be embedded into applications to enable developers to create custom experiences on top of those applications. VSTA enables independent software vendors (ISVs) to provide extensible software solutions with a minimum of effort.
VSTA consists of an IDE used by ISVs to build VSTA support into an object model they can expose in their application, a runtime engine, and an end-user IDE which communicates with the object model. VSTA fully supports the .NET Framework, including the VB .NET and C# programming languages. VSTA includes a software development kit (SDK) for integrating a customer interface object model into the ISV application and an end-user IDE incorporating VB .NET and C#. The application then uses the .NET Framework Common Language Runtime (CLR) for development and production.
Visit Microsoft's VSTA homepage for more information about the benefits of VSTA: Microsoft Visual Studio Tools for Applications 2.0