Ok, I found the final item. I just missed something silly.
In Vlad's tower, "Vlad's Amulet" is hidden in one of the walls. It's a gold amulet with a large circular red jewel. It protects you from fire, which is awesome! Magma men walk right through you, and you can even walk around in lava unharmed. (I never would have dared to try if I didn't already have Hell finished...) One of the final boss's main attacks is throwing Magma Men at you, so it's great for fighting him.
Edit: Yep, BanditBanks, that was it. Thanks!