I also think that the policy is there to not give them any legitimacy as a group with laudable aspirations. Casting them as a criminal gang of like minded psychopaths really pisses them off and allows us to ignore some conventions that we would otherwise have to observe when we have them in custody.
I'm not sure it would be supported by their doctrine to just out and out attack the Vatican, and these guys are all about doctrine. If the Vatican doesn't call for a war against ISIS, then it is doubtful that they would consider them a target. ISIS seems to have a bigger problem with muslim sects that they feel have gone astray (apostates) such as Shia, some Sunni, and Yazidi.I may have typed this to be interpreted incorrectly. I meant from an ISIS perspective, not from mine. For ISIS, it would be a feather in their cap to attack the Vatican.