The vassal system is out of whack. Alot of people here have made some good point. My experience with it is right along the same path. Two nights ago I was kicking major butt with a major tech lead. Everyone liked me except the Chinese, but I didn't care he was on the other side of the map.
A few turns later I notice everyone is p'd off. These are friendlies that I have been sharing technology with anytime they asked. All of a sudden they hate me and 3 of the major powers are vassals to the Chinese. They never asked for help, nor did they even need it. I still can't figure it out they were all the same size as he was. Then he pushes right past me to take the tech lead. I was forced to rush a navy, travel to the furthest point on the map, and raise his coastal cities. Of course it was full blown world war from then on out. So much for the cultural victory.
Last night I'm stuck on an large island fighting with Brennus. At the start of the war he's the major super power and I've only got 3 cities, that kind of suck. I declare and manage to take the first town. His capital is now right in front of me, so I juke towards it and he moves his units in. I sweep over and hit another lightly defended coastal town. Now, I'm going for peace right away because he can smoke me if he wants to. He accepts, the next turn he's a French vassal, WTH??? Now my trading partner owns him. What started out as a lame attempt to push my border back against Brennus results in the French being on top and on the bottom of me. Then, and this is what truly amazes me... Shaka freaks out because that's just what Shaka does he can't play well with anyone... he's also a vassal of the French. He's declared war on me while I'm at peace with the other two and is using their territory to attack me. The cultural boundary is pushed all the way up on my city. So they can attack me, but I can't even attack their stack!
I love the game but that's a bit much, and I would like one of the technologies to be prozac that I can trade with Shaka. He's a raving lunatic.