For the most part, I'm pretty happy with how vassals work in the game, providing a partner that will go to war and fairly but not completely openly trade resources and gold. I usually build up abundant resources and sell some off to my vassals, for what they can afford.

I'm usually glad to support the vassal with techs and resources while milking them for as much gold as possible, although one must be hesitant to share techs too soon because the vassal will turn around and trade them. I'm glad that the vassal is not a complete puppet, as that would make the game uncompetitive.

My problem with vassals is when going to war, they send in horse archers (for example) and will raid all improvements of the civ we're at war with. Sometimes I employ this strategy, but if I know I will take the city quickly, I want to keep those improvements. [For some reason, the AI never attacks these horse archers of my vassal civ as they raid the land, while if I were to do the same, I'd get attacked. buggy.]

I find the option to tell the vassal what city to attack has no effect on this and rarely seems to have any effect in any way. If there's one time I'd like to order the vassal around, it's when at war. They jump in at will and don't take direction. And I can't even fire this incompetence.

I'd also like to be able to flip cities to vassals in some situations. I like that the vassal remains competitive and will boost it's culture in some cities to run back the borders of my cities. Makes me want to set fire to their city, but it's fine that they remain competitive. It's fine if they want to attack a city to gain it when we go to war. But destroying all the towns in the city I'd like to gain is such a bugger. Makes me want to have the option to turn around and kill their units while we're at war together. Is there a way to mod the option to kill your vassal's units?