Ack, my Persian vassal broke off? Damn all power outages! I'll get the traitorous scum for that. I installed him there to frighten the Seljuks into making peace last time I was at war with them, not to become independent.
Germany is not in the last 'far away' from Russia; it's just across Poland. Poland is not only a nice small bite-sized nation, it's also all plains, perfect terrain for the savage dash of the Cossacks. And it doesn't take much Russian blood to make a given area Russian land. A drop or two is enough.
Oh, by the way, could someone post the save please? I'd like to take a look at the situation. I could maybe give it a purge while I'm at it, come to think of it.