As far as being able to Vassalize so many AI I would venture to say you can do that on any difficulty level. If you found a religion and spread it to your neighbors and have the same alignment as them its rare that they will attakc you unless you're significantly weak. Top that off with how easy it is to get a runaway score in FFH if you do a few key things and the AI has no ability to catch up thus leading to a score so high they voluntarily vassalize.
The last big 3 player .30k game we had I was able to win a tower of mastery victory within 50 turns by exploiting the vassal system and slavery civics. It sort of took the fun out of it. Even worse I would have been able to do it within about 9 turns if I wasn't missing one mana type.
I forced Tatsunke, Sandalphon, and Faeryl to all give me all their mana. I got off to an enormous lead due to using Loki to steal 3 of Faeryl's cities (take that Loki needs buffing thread! lol) They all volunteered to vassalize at which point I took every unique resource and mana they had resulting in my cities getting something ilke +9 happy and +18 health per city as well as letting me rush 3/4 towers. After Rites of Ogma finished (Celestial compass didn't halve it maybe because I had already started it?) I built dimensional mana then rushed the Alteration tower and finally the Mastery tower. gg
Can vassals become independent again like they do in BtS? I noticed the (admittedly somewhat confusing) mouseover stuff from BtS about % size in relation to master, etc., don't appear, and wondered if that meant they are no longer relevant.
One vassalized civ in my first game appeared to be recovering rather well, to the point I thought it might become independent again.
BTW, is "Aggressive AI" in FfH similar to that in BtS, where it's really what should've been the "default" AI, as opposed to many games, where "Aggressive" just means endless, ultimately pointless, waves of kamikaze attacks and the like?