Right now I'm in the middle of my third game with the closed beta, this time with difficulty level Civilization (6/7) after 2 games with difficulty empire (5/7).

In each of theese games I had a war with my neighbour to the south, the green AI player (I think the name is Vlad), very early, during ancient and classical eras.

The AI has more units than me, but playing defensively and choosing good terrains I've always managed to win lots of battles and ultimately the wars by forcing surrender.

In this 3 wars I've almost never led my armies on enemy land, only once I've occupied a city and never even laid siege onto the enemy capital. 

Yet still every time the green empire ended up being my vassal, giving me lots of resources and gold.

Now, i really like how war is handled in Humankind, from strategy, to battles to war support mechanic. What needs improvement is not war, but peace.

In my opinion vassalization cost should be higher and also possible only if the enemy capital (or at least the majority of enemy cities) is occupied. I've played other games where you can vassalize your opponents, and I'm very glad that this possibility is available also in Humankind, but only in Humankind I could subjugate an enemy empire without even sending my troops outside my borders (just happened 1 hour ago, with level 6 difficulty).

Having vassals is OP because it's OP in the real world, what is broken is the way you obtain vassals after wars.