If religious tensions got to the point where the Vatican was bombed then you already have reached ASBland. I don't think that Al-Qaeda would want to bomb the Vatican. Doing so anyways would contravene the teachings of Islam. But it's an interesting possibility - albeit a depressing one. The Catholic Church in the aftermath of such a bombing would be in sorrow and so would every decent Jew, Christian, and Muslim in the world. They would also have to rebuild the Chapella Sistana - which took hundreds of years to build. The restoration project would get donations from Catholics all around the world. At the end of the day the Catholic Church may yet emerge stronger. I see Francis as a saint and someone in line with him theologically taking his place. I can see Al-Qaeda being more hated than ever before, even if it was done by a rogue branch of their organization.