So, is there any way how I could anticipate if after a succesfull war I will be able to vassalize another empire so I could decide if to war them or not?

In my last game I needed Oil and I was more advaced than my neighbours. One of them had many undeveloped oil deposits so I decided to invade him and try to get the territories with Oil or vassalize him.

After a super succesfull war (no defeats on my side and two conquered cities) I forced him to surrender only then to see that I could not get any of the territories with Oil (something that I could anticipate, given that I wasnt adjacent to any of them) but I could not reach the war score needed to vassalize him. So everything I got was just a lot of gold (that I did not needed).

This is something I also have a problem with, because sometimes you will get the +100 extra war score from winning and sometimes not so it is very hit or miss whether you will be allowed to vassalize your oponents.

Oil and Horses in Humankind are reversed from actual Earth. In the game Horses are all over the place instead of being plentiful only in a certain region. In the game, Oil comes from whatever part of the world it evolved? I guess? But they do want to make you go to war for oil because that's part of our history.

The easy way around this is to play as the Chinese and use your Merchant ability to just magic an oil extractor into existence on top of some other empire's oil. While you're at it you can magic extractors into existence on anything else you're missing, but only once per 10 turns.

As far as I can tell, if you want to vassalize an opponent you need to take as much territory as you can even if you didn't want it (don't accept the gold!). Then go to war again as soon as you can get the support. Don't forgive any grievances. The second war you'll probably be able to vassalize your opponent.