I've finally gone about getting Vassals in my campaign as Venice. I managed to get England, the Mongols, and Spain as Vassals.
Now this is toward the end of a long campaign, but still, I noticed something, on accident.
When you are at war with someone, and have given them a solid pounding, and it seems like they should want out... but you keep seeing "Very Demanding" when you click Ceasefire? Just try giving them something first. All I gave was Map Information, as a gift. That broke the ice. Then Ceasefire read as "Very Generous" and Vassal as "Balanced". They are yours!
So that seems to be the trick... you have to open negotiations with a gift, any gift, before you can see their real attitudes. After that everyone is falling over easy. My Reputation? Mixed. England had 6settlements, power at Supreme level, and hold of Invernesse, Edinburgh, and other cities I didn't border. I still got them.
The Mongols are even easier. Once you destroy their last Settlement, and they go into "Horde Mode", they are VERY vulnerable to becoming a vassal. Just give them a Settlement you don't mind having them look after for you. I gifted them Sarkle, then demanded they be a Vassal. They have to have land before you can ask that... and to be honest, setting them up as a client state is a lot easier than chasing them around the eastern steppe for 20 turns until they run out of family members!
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