Although the Vatican is an independent nation, it's in the middle of Rome. A major bombing of the Vatican is likely to kill Italian citizens who are working or visiting. It may also kill people from all over the world who are visiting as tourists or pilgrims. I have a hard time believing it would not be treated as an attack on Italy and be used to justify some sort of military response by a variety of nations, regardless of whether or not the Vatican issued a statement against revenge. Also, while the official Vatican position my be against revenge, there could be a groundswell of demands for revenge by Catholic laypeople, especially in places where turning the other cheek is not always high on the list (after all the IRA was not known to be a quiet bunch, and the US is always ready to stage strikes on ISIS and other terrorists).
On the other hand, I see absolutely no reason ANY western government would use this to justify an attack on Mecca.