As it stands, if you are a vassal of someone, and a foreign power tries to take over your land, you aren't personally considered at war with them so you don't get the normal bonuses of being in a defensive war, so far as I can tell. For example, in my game as the duke of Anatolia, the Fatimids jihaded the kingdom of Anatolia. It seems to me that as a duke in that kingdom they are trying to take, I'd be fighting tooth and nail to keep them from usurping my lands, and my vassals are all under the same threat, so they ought to be supporting me for doing it - however, I currently don't get any levy reinforcement bonuses for defending, and my vassals get no opinion boost for defending against heathens (and the malus for having levies raised keeps going up). I think this should be changed. If it was, it would also help curb some of the blobbing in the Middle East, since the Fatimids would have a harder time simply holy warring all the land around them if the dukes they were trying to take over actively fought them as well.