This is a test about the trade between the master and the vassal, under
semi-isolation.
First, started a game in Fractal map and hand-picked Tokugawa as one of the AIs. I chose Toku for the vassal AI in this test, because Toku is the most stingy AI about tech trading - he only trades techs that 100% of other AIs know. If Toku is willing to trade a monopoly tech, all the other AIs under similar circumstances should be willing to trade it.
Next, regenerated the map until the human player (I chose "random" and the game gave me Shaka) and Toku were semi-isolated. In WB, changed the tiles around Toku to unpassable peaks/mountains or oceans, so that Toku couldn't expand.
As shown in the mini-map, Toku and I were in a small continent. The 5 other AIs were in another big continent.
Unfortunately, I don't know how to give cities to a player, so I had to self-build some settlers and send them to found cities (to increase my land area). Gave myself 10 infantries in WB (to increase my power). Also changed the diplo with Toku in WB to make him Pleased:
Once I became Toku's land target and got Feudalism, Toku was immediately willing to peace vassal:
After I clicked "sounds like a deal", Toku became the human player's vassal:
Gave Alphabet to myself in WB (to enable tech trading), and here is the result: Tokugawa is willing to trade IW, Archery, and Maths, even we're semi-isolated and he's the only one who know these techs:
(on a side note: in 475BC, going for Maths and IW but not Agriculture, no Pottery <--typical AI Toku's tech pace at Monarch
)
Toku is willing to trade these monopoly techs, mainly for the reason below:
So, this test to some degrees confirmed @ f1rpo 's theory.
The trade between the master and vassal can be dangerous, because some AIs are stingy about tech trading but they're willing to become a peaceful vassal at Pleased. The peace vassal makes these AIs trade unlimitedly with their masters. For example, AI Huayna Capac is willing to be vassal at Pleased. If HC is semi-isolated with Hannibal, and HC becomes a voluntary vassal to Hannibal, it might create a monster combo.