In diplomacy, several faction specific factors play a role: Whether it is a shadow faction, whether they are of the same religion as you and the current standings/reputations.
However, as you can see those aren't really specific to any one faction. Other variables do depend on monetary value and have been modified to make diplomacy more useful, but I didn't want to make florins too useful either, where you can buy half of someone's empire for 100k. While this isn't inherently a problem with humans (you can reject the offer) the AI just works by balancing numbers and will accept any offer (save military access and sometimes map info - seems to be an engine issue that doesn't let me fix these to display as the others to the fullest extent) that is rated generous or better.
Oh, and making factions your vassal only works if they want to be your vassal, i.e. are not a shadow faction and don't have a lot of allies that are all at war with you. You also need to be bordering their regions and have significantly more military prowess than them and their allies combined.
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