I haven't done much with vassals since updating to the revisions that introduced the Foreign Policy civic(s), so I don't know if this is "working as intended" or something nutty going on, but I know I've never had such a severe drop in income simply from accepting a vassal before!
So Darius I came to me and asked to be my vassal. I thought 'sure why not' and accepted him. Cash dropped slightly, but it wasn't a big deal. I could be running at 60% income just fine. I go about my business, complete
The Silk Road and am trying to secure a few additional resources and some space-filler cities
(Darius having snuck a settler behind my borders while I wasn't paying attention. Serves me right for being so slow
) when Kangxi comes and asks to be my vassal too. I actually kinda like vassalizing the AI's, so I went ahead and accepted him as well.... Only to notice my income plummeting to catastrophic negatives!
City maintenance had jumped up from 50'ish to 210+ just from vassalizing him!
Now as I said, I haven't been keeping up much with vassal development in the mod lately
(I've actually been a bit inactive. Just started up some college classes and they've been eating a lot of my time) and I haven't exactly had time to experiment with how they work alongside the new civic policies, but this was an eye opener for me!
He's running
Appeasement and I'm still running
Isolationism, so maybe the two are conflicting somehow, I don't know - but my maintenance costs skyrocketing... I can't remember, but is that supposed to happen?

I certainly don't remember it being that severe before, but then again a lot has changed with the mod since I last accepted multiple vassals. If this really is 'as intended' though that's a major hit to how I usually play the game (going nuts with vassals) eep!
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Now granted, Kangxi's not the best choice of civilization's to vassalize at this point
(He's been under the wonder-rampant Atotoztli's rule and military powerhouse Deganawida's rule already, and has a city squeezed in on the former's homeland) so I can always decline his offer the next time around, but overall it does make me a tad worried. I love vassalizing the AI almost as much as I love crushing them - via military or culture - so this would definitely hamper my playstyle if I end up having to pay maintenance for THEIR cities along with my own