In a four-player game, my brother's second game, and he accepted a vassalization demand from a powerful hegemony while I wasn't around. Hilariously about a decade later we figured out that he was the Hegemony president. After disbanding their fleet and trying to pass some intentionally bad laws, he decided to actually keep it.
One problem, as anyone who has tried to play as a vassal might know - the AI will absolutely integrate player empires. I just wanted the CK2 experience
And in what I can only assume is an egregious bug, the target received no warning whatsoever. The rest of the galaxy gets a popup when it begins, but not my brother. Obviously we explained the situation, but I've been burned by that and it sucks a lot.
Then in a 100% definite (and previously reported) bug, there's no way to forcefully escape vassalization if you're in a federation. You can buy 10 favors and ask, but even after a flood of envoys and wonderful relations the acceptance was a flat 0: -50 base, +50 from favors. Didn't work.
But it gives a casus belli to rebel, right? Well, sure - except you can't get casus belli against federation members. Ever.
We ended up having a player rejoin as the overlord and release my brother's empire, then rejoin back. My brother remained the most powerful member and led the Hegemony with a reasonable fist, keeping good relations with our neighboring trade federation, in peace and prosperity.
until the Khan woke up directly between us