See this document: How to: Publish Events that Conform to .NET Framework Guidelines (C# Programming Guide).
There is no compelling reason to follow those guidelines when you are making a game. Make your own delegate type that takes appropriate types (either existing object references or value types):
public delegate void DidSomethingHandler(MyThing what, Vector2 where);
public event DidSomethingHandler DidSomething;
Or, if you want to be especially lazy (protip: you want to be especially lazy), you can even just do this:
public event Action<MyThing, Vector2> DidSomething;
(If creating your own delegate provides better documentation or will be used in many places, do that, otherwise just use an
Footnote: When are those event guidelines applicable? When someone else will be consuming your code - especially in versioning situations. See also: What are the benefits of having events conforming to Net guidelines?.