How to make AI factions break THEIR alliance?

[wimlore]

I'm having trouble again... I have these two neighboring countries who are allies. One is a vassal and the other is not. There is no way I can make the ally break his ties with my vassal, or vise versa. I mean, im sure there IS a way, but I'm having trouble finding out how?

Im thinking it has a lot to do with:
-whether or not YOU are the agressor in the war.
-if your reputation is high enough.
-if your relations are high enough to make an allie choose your side over the agressor's.

I've made sure I was all of those things, but my vassal still chooses to team with his allie ;( Do AI alliances ever break if the player is involved? Theres no diplomatic demand for them to break relations(like there is in Knights of Honor ) I hope someone knows the lowdown on this

[Jiub]

If you piss them off by constantly crossing their boarders with your army, but not attacking. It`ll drive them mad and will probably attack you eventually, thus their alliance with your vassal will break.

Edit: Making propositions that are very demanding with your diplomat also decreases your rep with that faction.