Could someone explain the excuses civs give for not becoming vassals?

[DanF5771]

Anything about "We can't do that.".

If by "We can't do that." you mean "Sorry, it's out of our hands." I can tell that an AI (Joao) who is currently at war with a Master(Justinian)+Vassal(Charly) will return that denial when you try to negotiate peace between this AI and the Vassal.

And the "We are afraid of your enemies!"? I noticed a wierd behaviour from Sury using that reason because he was afraid of icy Ragnar (in other words backward and weak) and the moment I killed two of his units in the BFC of a city, suddenly Sury agreed to capitulation.
I wonder about immediate thread factor. I remember DanF said something about barb(or enemy units) in BFC impedes AI city liberating. Now, I am starting to feel the same for capitulation. May be wrong though.

I'm not 100% sure, but I think you could have killed these 2 units anywhere on the map and still would have seen the same behavior of Sury. Sury most likely fulfilled the isLandTarget-condition for Ragnar and by killing the 2 Viking units you probably put Sury's power >= Ragnar's power... So it shouldn't be the result of immediate threat (2 units in BFC) but you are still right in general, as factoring in local threats is what the isLandTarget-stuff is supposed to handle.