So, I noticed that allied and vassal armies now keep piling up on my armies. Thus not doing anything useful and increasing (causing) attrition.
Keeping a stack or two around with 1k men makes the AI much more likely to follow those around, leaving your main stack alone. This makes it very easy to direct your vassals or allies while simultaneously fighting with your main stack.
Guess I'm alone in wanting my allies to do their own fighting and to be able to hang back and relax... I don't really enjoy the fighting part of wars (HoI is much better for that!).
No offense but what do you do in this game besides fight wars? I can see how it's convenient to have your vassals fight for you but that's only really possible if you have large vassals.