I was wondering about vassals as well. I played Kong Rong in my first playthrough and people constantly declared war on me even if I had treaties and decent relations with them (even 'trusting', or 'honourable' personalities).
So I thought I would try again and try to make use of vassals instead but I never had much luck, the decay on positive relations is extremely fast, I went from about 200+ in one case down to about 30 in probably 10-15 turns, and then of course they rebelled (can't say I blame them though).
Which of course leads to the other comical thing about the game is the vassal blob that the AI accomplishes. I went out of my wei to take out Yuan Shao since I thought he was the vassal master but another AI faction just picks up the slack:
Sun Jian and Sun Ce both died off (I guess scripted events) and so Lady Wu took over and start vassalising everyone (resistance is futile) just like before, even Liu Bei who was strength ranked 3 (Lady Wu was 2, and I was 1 and I was in a coalition with Liu Bei, which just doesn't matter apparently):
so there is a huge plus for 'treaties with friends' not sure about the benefits. Even Yuan Shu who didn't have good relations to begin with shot up to massive relations in a short period of time
The embedded spy shows that she seems to pretty weak overall, so where is this power to vassalise coming from? Is it an anti-player effect or just a bug?:
I say that because for me, it seems difficult to get them to agree to it and then even harder to keep them from not going solo again: