@ ntwfan
Taking Vienna can be a challenge, as well as create further ones.
It is a capital city, and you will probably have a public-order crisis, putting down rebellions and whatnot. However it does make Austria make peace. If Austria and Prussia are allies, then taking Vienna to make peace is a silly idea, because Prussia is meant to declare war on you one turn after, along with it's allies. I would suggest you make them break allies.