As
@MacGruber already said, you don't need anything special, just Notepad++ will work.
I usually use Visual Studio because it will highlight VAM's classes (I can't find the tutorial where they show how to import them, but there's one out there).
To learn how to code first you'll need to know some basics of C# and Unity.
After that I recommend you the tutorials
"Hello World - plugin basics" and
"Hello World - plugin basics pt2".
To learn more in depth I usually look inside the VAM sourcecode using dotPeek (you can see all the classes/functions and how they are called).
Another good aproach to learn how to make plugins is to look others' similar codes to see how they do it. I wanted to add that you don't need VAM to "unpack" the .var, if you have WinRAR you can unzip it directly.