Something that happened in my latest game--not sure if it is a bug or just overlooked as it's an uncommon occurrence. Playing as JFD's Great Britain (Victoria), I've vassalled The Huns and Persia. I declare war on Venice, but didn't realise he had a defensive pact with Denmark and Germany. Not a problem because I have a Persian buffer between my conquered cities and Denmark--the only thing is, Persia and The Huns are only at war with Venice (and it's city-state allies), not the other members of the defensive pact. I can see that being intended, but to my mind if any nation declares war on me for any reason then they're also at war with my vassals--in an earlier war Portugal attacked the Huns, for example. So it was unexpected for Persia and The Huns to only be "partly involved" in the war. I did try to talk to Persia to get them to declare war on Denmark but their name was greyed out with the text "diplomatic events prevent this action from being taken." I'm assuming that's because you can't get your vassal to declare war without being involved yourself, which makes sense--just thought I'd mentioned it in case there was some other diplo weirdness involved here.