Rumor control: player vassals have not been coded to do nothing. If they have the power and potential target they can declare war on their own. When I played a King of Ireland game last week, one of my dukes decided to attack Scotland on his own and later try a few attempts of crusading in the Levant.
Thanks Captain for allaying my fears. I am glad to see this feature is in the game. I was just surprised to never see it in my games. And I still think it needs to be tweaked because if a vassal has very good relations, they should be use their levies to support my wars. I can understand if we had bad or minimal relations that he would do whatever is in his interest, but the fact that they never raise their levies even if we have 100 relations concerns me.
And the other issue that should be added is whether or not we a liege will support a vassals war or not, there should be a choice to support them or leave them go on their own. Also in a game as a duke of the byzantines I went to war with Aleppo, assuming i would get some help, and even though it showed the emperor was in the war the emporer did absolutely nothing while all my lands were taken.
Edit: BTW I am not trying to bash the game, and hope it doesn't come off like that. I love this game, best game i played in a long time! I just miss a few of the features from CK1 that I assumed would be in the game.