Capitulation to an uninvolved 3rd party?

[Wilphe]

When you are war with someone and while they will not capitulate to you they will vasslise themselves to an uninvolved 3rd party who ends up at war with you?

what is the AIs decision making process for both the vassal and the master?

 

[PPQ_Purple]

For your enemy: "This guy is my friend, if I surrender to him he won't make too many demands and will join the war on my side."

His friend: "This is attacking my friend, now I get a good excuse to bash him and perhaps get some loot too."


In other words, the AI fighting you makes an informed decision to do something that will both deny you, the human it hates the spoils of your war and possibly allow him to defeat you afterward anyway. He can always quit the vassal deal later since its a voluntary vasalisation and not a capitulation in war.