VSTO is great for creating a user interface inside of your Office application and adding custom code, event handlers, databinding, templates, and such. However, if you don't want to launch the Office application (WinWord.exe, Excel.exe) and run your code inside those processes, and you would rather manipulate the files themselves (mydoc.docx, myspreadsheet.xlsx) then your best bet is to load the Open XML SDK onto your developer workstation.
The Open XML SDK is a great tool for automation when you want to quickly read or write to a document without using the Office application nor the Office Object Model.
I hope this information helps you make a wise decision as you try to choose the right tools to solve a problem.
-Christin Boyd, Program Manager, Visual Studio