does it still work ?

@jowe01 - You bring up a good point.

I've updated the mod to better account for the Vassal paying for maintenance. (Each vassal is hit with a 5% maintenance fee, but only on their own ships.)

@MayhemX8 - - Great suggestion!

I've updated the mod so that any ships/fleets the vassal owns at time of vassalization are immediately transferred to the overlord.

@Caveman Grug
@GnoSiS

Thanks for the suggestions, guys! I've updated the mod based on your suggestions. (See updated description.)
This is now working for 2.3.3.

With how passive vassals are right now and how stupid the AI is. I would suggest to cancel the -40 the vassal gets altogether as per caveman's comment below. I assume you can't mod the bonus to be a percentage of the subject hence the flat 40.

I've played with this for a while, and I believe it to be the cause for why my vassals are rendered essentially inert: I tend to vassalize smaller empires, which means that I am taking almost all of their fleet capacity by taking 40. However, often I win the war without ever destroying their existing fleet, so the existing fleet is at or over the naval capacity, this causes them to economically spiral into obscurity, unable to scuttle their ships to save their economy. Just as well, by vassalizing someone you are permanently crippling them to be unable to advance, so this has late game and mid game balance issues. I would recommend changing it to a percentage of their naval capacity, say 40%. This is less likely to break their AI. And maybe also put in a wargoal modifier that makes vassalization unlikely if they still have a fleet. This way, the AI will not be vassalized into a peace which will kill their economy forever, since the AI seems incapable of scuttling ships.

I have a sort of better idea Terberlo, what about creating an alternate version of this mod which lets the Overlord take control of all the navy fleets and armies of vassalized nations during a war?

I say its balanced because while you do get entire fleets literally for free, you can't design them to your specs, so they're really just "all-purpose" fleets. Great for throwing at the enemy to soften them up for your own highly specialized and meticulously engineered personal fleets.

I suggest this because I like the idea of a Feudal Empire with enough strong Vassals that it only needs to maintain 1 fleet of its own, and trying to direct vassal fleets is like herding cats.

Logically, the vassals Naval capaity would then need to decrease by 40 and his upkeep increase by 2% (well actually by more, given that the overlord will usually have a bigger fleet than the vassal). Is that the case?

@Kepos - I like that idea! I did investigate that option for a while, but gave up due to complexity. I'll look into it further.

The idea is interesting but fixed capacities might be a bit OP from my view.

How about a dynamic counter, depending on the size and technical background of the vassal?

There is a mod Feudal Expansion Policy to allow vassals to expand, which is cool and I can see very nice cooperation between those two.