In a recent game I was Monty and Mansa was my neighbor. To my surprise about one third of the way into the game Mansa offered to be my voluntary vassal. I guess he either liked my big smile or wanted military protection. I accepted the offer and for the rest of the game had this monster techer as my vassal. He was friendly to me and so would trade the techs he researched, but only if I would trade him equivalent techs in return plus the usual premium. It was as if I was pursuing two lines of research at once and helped me win the game much faster. I later attacked and vassalized a different AI who was unfriendly to me and would trade no techs. This made me think what are the optimium strategies for the human player in managing vassal research. If there is a thread or an article which has addressed this question, I would like to know how to find it. If not, this thread might be the place for working out the best principles. Am I correct that vassals can trade their techs to other AI's without the human player's approval. If so, this complicates with to do with a friendly AI - like Mansa in my game - because he may trade the advanced tech he has just researched to other AI's and not just to me. How should one best manage vassal research and what variables effect how to proceed?