It looks to me like you might be having an issue with permissions on the variable. If it's in C# then it may need to start with **public** (e.g. public Rect Window=new Rect(...); ). You might also need to put **static** in there if it's stored in another file to the one your trying to access it from:
C#: public static Rect Window=...
Js: static var Window:Rect=..
If not try storing a reference to the Rect before using it:
var theWindow:Rect=screenPresets.Menu.Window;
v_MenuWindowSize=Vector2(theWindow.width,theWindow.height);
Hope that helps! :)