I've never had a game where Tokugawa traded a tech with me. Whenever he's in a game he usually falls further and further behind till someone puts him out of his misery. I always kind of assumed he doesn't trade techs with anyone at all, including the AIs.
In my current game, Tokugawa is my vassal. We're at war with Mehmed and Augustus. Mehmed has twice as big an army as everyone else but I got first to rifles and cavs. It occurs to me that if Tokugawa doesn't trade with other AIs then there is little harm in gifting him with the techs. Tokugawa with no tech disadvantage would make a pretty good vassal.
In my current game, Tokugawa is my vassal. We're at war with Mehmed and Augustus. Mehmed has twice as big an army as everyone else but I got first to rifles and cavs. It occurs to me that if Tokugawa doesn't trade with other AIs then there is little harm in gifting him with the techs. Tokugawa with no tech disadvantage would make a pretty good vassal.