So many dead lads from both my countries fighting American wars, for zero gain to our peoples. That's vassalship, not an alliance.

Allies will only help you if you are winning the war. Vassals will also help if you are losing.

Whereas vassals are countries that you draft to help you in war?

There's certainly a spectrum between allies, vassals, and directly controlled territories, but it doesn't necessarily need to be about sending troops to help in war.