random question, is there a way to stop your vassel going to war another civ and so causes a unwanted world war. or maybe, is there a way to get rid off one of your vassel so it would drag you into war.

I'm pretty sure vassals can't declare war.

apparently in RFC they can, it got me by surprise as well.

Just in my game when I played as Vikings I vassalize England by beating English in colonizing the Scotland and Ireland. England, as usual, colonize the east coast of NA while I colonize the central parts. Then America spawns. Probably English reject them when it comes to the transfer of cities and declare war on America. As England's master I was forced to do so. With their superior strength they take a couple of my cities :(

Yes, I'll second that, it's happened to me with the lasrt version.