Yep. I had four stacks of roughly 20,000 troops apiece. I was thinking that with any luck whatsoever, I would have been able to take out at least half of the khan's army through strategic maneuvring. So much for that. The entire kingdom just turned over all at once before my troops even got to the front lines.
You were probably forced to accept peace due to -100% warscore. Haven't had the same problem. Managed to hurt the Il-khanate with a failed offensive war successfully. Got lucky with one of my vassals waging a holy war against the target of the Golden Horde's first invasion. They started out with only 3 baronies, with the city and bishopric in the provinces controlled by my vassal. Several wars later, where for the most part I was significantly outnumbered, I finally managed to destroy their last army and promptly annexed their last holding. I hopefully assume that this means they're forever destroyed.
I recently noticed that Hungary, which had been making strides in bringing catholicism to the Rus, became a vassal of the Byzantine Empire through some war. There's now a massive bloc of orthodoxy thanks to the Byzantines, Russians, and myself (Georgia).
How do you become a vassal of an emperor when you're a king? I see no option. My original plan with this game was to become German, which I did, and become the Byzantine Emperor in order to convert as much of the map to German culture as possible.