I posted a Japan with most of East Asia a while ago. But in the mean time I hardly ever had a coalition. Nothing global, just minor ones here and there. The key is not to take territory directly.
I declared at least 12 or so wars against Ming. Ming had engulfed Manchus in a PU back then. I took just two provinces, ones with Xi and Manchu cores in the first war. Release Xi and Manchu, and declare again when the truce is over. Return cores to Xi and Manchu, take just two more with Shun and Zhou cores. Repeat. Repeat. Repeat. Repeat. Eventually Ming disappears since ALL of Ming's territory is returnable, and I don't take aggressive expansion for returning cores to my vassals.
It gets easier as you play more, and preferrably in different regions. Because then you'd practically know where nations and their dormant cores exist. I usually take a slice off, release vassal and return, but simply releasing a big chunk (say, Kazakhs from the Uzbeks), vassalizing the original country and returning their cores back works better in the late game when the core expiration begins.
Further, once you're getting stinking big, you can use your tax base and diplomatic reputation taken from ideas to diplovassalize smaller nations in the same religious group. In that Japan screenshot, I was Shi'ite. I vassalized and annexed a bunch of Muslim nations in India and the Middle East, with very little direct conquest in the region. Since the AE scales with size, if I decided to take territories directly, even taking a couple of provines could have incurred a coalition against me.
Would you still sell land to your vassals if it didn't get you a free core later on?
Of course not. That's the gamey bit. You're exploiting the mechanics to get something which otherwise costs quite a lot of time and admin points... for free.
I wouldn't call the dev acknowledged mean of expansion as an exploit. Gaining cores is what is meant to be: some of the built-in things like Austria's PU missions against Bohemia and Hungary, Poland's vassalization mission against the Teutonic Order, and the Iberian Wedding are all based on the idea that you gain cores on the annexed/integrated subject nations. It's just the way this game is played.
Selling lands to vassals is what AI does as well. Seriously. I was playing Perm, and my overlord Muscovy sold me a province from Kazan. I took it, started coring it, and soon after Muscovy sent me a gift and started annexation process. As a player I can stop the annexations any time I wanted, but still I thought it was cool that the AI does what a player does.
"Muscovy offers us the province Perm for the sum of 50 ducats. Do you accept?"
See, Muscovy can still core this. A landlocked province connected by a vassal territory can be cored - I'm sure that is the case since I did that just last night. Muscovy simply wants to reduce overextension, and they probably don't want too many Sunni provinces around. Seeing that the province has the same culture as my primary (Uralic), and since I already put a claim on it, Muscovy is feeding me.
Likewise, sending gifts and offering vassalization is not restricted to humans either. The AI does this. Austria just goes around diplovassalizing all over the HRE, but it can happen even to a human:
"Zhou offers us to become their vassal. When we accept, we become their protectorate. In return we must give 50% of our income, and we are obliged enter alliance only with them."
The AI doesn't do this as extensively as humans, but my point: if AI does it, it's not an exploit. If there is any problem with "vassal feeding", it's a fundamental design problem, not the users exploiting the game mechanics. You just don't see these kinds of stuff if you're playing big nations, but try playing as a vassal. You'll see the AIs are pretty smart and humanlike in their strategies. After seeing these kinds of stuff being done to me, I can't buy any arguments that doing these things are "gamey exploits" at all.
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