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?

Yes, by definition, a vassal owes allegiance to their lord, which includes military service.

But you can’t really take a feudal term and apply it to modern politics. Taiwan doesn’t resemble a vassal in any meaningful way.

I think the idea is that a vassal state (operates as though it) owes allegiance to a stronger state, while governing itself (more or less) independently.

And the stronger state exercises a lot of influence on the weaker state, especially in terms of foreign policy, and potentially provides it with protection from other strong states.

Not sure exactly how much this would apply to Taiwan in particular, but I think the idea in general can be meaningful in modern politics.

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.