I went quite far militarily in my last game, and I beat up 4 or 5 AI and the "first" to surrender were the Zenith. That's actually the first to surrender ever in my games - but I didn't generally focus on beating them.
As far as how AI work, for most there seems to be a "core" building which enables them to build other stuff, and "small" structures, basically everything else. Some seem to have multiple of the "core" buildings (I think the "core" for the Spire is the "mother" ship, but not sure). Anyway, for me there is two cases:
- either the point is that vassal become your allies in some manner, so they NEED to be able to build. In that case, I'd put the surrender condition to "core building destroyed at 80%" or enable them to rebuild their core building (at your discretion, by some kind of diplomatic demand) once vassalized.
- or you don't need them to be able to build. So once the core buildings are down, they surrender.
So basically, depending on the cases above, I'd suggest "once core buildings are destroyed" + proposal to have them rebuild some "vassalized" version of their core building in order to make them "usable" allies.
Next proposal would be their behaviour once "killed". Chasing every last underground / aboveground buidling will get old kind of fast, as would shooting them to get the "resource" you want. I'd propose to have complete vision of your vassal's buildings, and the ability to destroy their building with the "destroy" action. Yes, I know you can still shoot them while vassalized, but I'm a benevolent dictator
. I don't want to systematically shoot them. Bulldozing their homes and farms is sufficient =).
Another idea would be to "tie" their core building as a necessary part of their "existence". For example, once it's killed, all buildings would decay, unless you allow them to rebuild the "vassalized" core building (1% cumulative damage to all buildings / turn).
For this idea to work, I think it would be necessary to make the core building much more offensive than now - territories seem to be able to help there. Currently their (very low) range is a liability - core buildings might also need multiple attack and attack types per turn. The idea would be to make them "capital-like".