How do I figure out which of my vassals are at war with each other?

I want to change my succession laws, but I can't because some of my vassals are at war with each other. This has been the case for many years, and I've changed crown authority to medium without affecting this situation.

How do I figure out which of my vassels are at war, so I can transfer their vassalage/murder them/excomunicate them?

I've emperor of the HRE, so I have ~76 vassals, some of whom are at war with outside forces. I cannot for the life of me figure out which vassals are at war with an outside power, and which vassals are at war with each other.

There's no way to get a quick and easy summary of this. There are two places you can find this information, however.

You can click on your vassal's provinces on the map, and the infobox on the left corner will list what wars they or their liege are involved in.

Alternately, as stated in the comments, the ledger has a list of all active wars in the world. From there, you have to sort through the list and figure out which ones your vassals are part of. This probably is easier than the first method for large realms.