According to the CK2 wiki, which I imagine refers to the default un-modded rules unless otherwise specified, it states that "if you're in a different religious group than your target, your target is a heretic (of your parent religion, so Catholics can't attack Monophysites as they're a Miaphysite heresy), or you are heretic, you can declare a religious war." I would guess this equates to what you are calling vanilla holy war requirements. Add to this that I meet all the Decision requirements as noted in my last post and I still can't declare war on a non-neighbor Muslim as a Catholic because I don't have a valid CB. Where is this requirement coming from if it's not in the Decisions or the regular game rules?