Is there any way to tell whether an AI is willing to capitulate to another?
I'm currently decimating Shaka and will switch to his freshly-ex vassal, Montezuma afterwards, and at some point I took peace with him (before redeclaring) and in that time I've asked Darius, who's also very strong economy wise, to attack Shaka, thinking that this will help severely thin the forces of all three leaders involved. (And it did

)
And at some point I got overexcited and asked Ghandi to join in as well, despite the fact that there's nothing he will ever do in this war, other than perhaps lose a few fishing boats.
I'd like at least Darius to keep losing forces and to keep thinning the forces of Monty, but I don't know for how long I can keep this up without Shaka vassaling to him. Shaka is already willing to capitulate to me, btw, but I don't want him as a vassal, I want him off the map, to get rid of the motherland happiness penalty and to get rid of a leader who'll be furious with me for the rest of the game.
Is there anyway to tell when Shaka vassals? Or should I play it safe and ask Darius and Ghandi to settle for peace?