For example: a city that has 10 pop points currently saves you paying for 1 unit (empire wide, 10 pop = 1 saved unit cost), but such a city costs you 0gpt at no cost, 1gpt at low cost, 1.6gpt at medium and 2.2gpt at high cost.
Source is here
Two cities with lower pop in each city would cost you even more gold, but even then, how many size 20 cities do we see? Even then, best case, save 2gpt in unit costs but pay 0/1.8/2.8/3gpt? Best case scenario Vassalage, at low cost, can just about match Nationhood at No cost which means it's 2XP versus drafting, +25% EP and barracks happy. Right now it's at medium cost, 10 pop per saved unit plus 5 free units.
The civics themselves do not need the functionality rebalancing, but the costs do need fixing. I'm not saying I know exactly how the costs should be aligned, just that Bureau at No cost is the obvious starting point.
PS. Charriu has a bug fix for events so I want to post a new mod version by tomorrow at the latest