When they say they will fight along side you, they will join the offensive war. That is they will be in the war themselves as allies in the war instead of giving you extra levies.I found a bug with the mod. As a king, my vassals cannot join the war directly. The event fires and it says that they will fight alongside me, but it doesn't seem to have an effect. They don't raise any troops or give me any levies. The version where it says all of their troops are ready for my command works though.