Right now vassalage is a reasonably good method of acquiring basic power since it gives you automatic access to logistics and their troops will no longer harass your empire, but overall in my games they always provide very little economic benefit, other than simply gaining access to their resources, which is often SIGNIFICANTLY less beneficial than trading for resources since you don't earn any money from trade routes, and the vassal tribute is tiny. I interpret that this is meant to be a trade off between vassalizing a nation and leaving it independent, but it honestly makes vassalization abysmal compared to just taking land or trading resources.

Can it be changed so that when a nation is vassalized, trade routes are automatically made for free between all of both empires' imports and exports? This would make vassalizing more nations actually profitable if you have the military power and influence to hold it. 

Oh, I do like this idea. The simple act of making resource sharing between liege and vassal take the form of "automatic, forced trade routes" is a nice boost to make vassalization worth it instead of just taking everything in victory.

+1

I mentioned this too! Cultures like Venice and trade related bonuses are basically worthless by end game because there's no trade - either everyone hates each other or they are vassals. I showed the end screen of gold from trade and it goes to 0 in every game.