I've found she favors any old helmet you give her to hold, but looks best in Glass. She wears the light armor Dawnguard guantlets and boots (which I found very ironic), but not the Royal Vamp armor (or any version thereof). She's swing a shield (if I give her one to hold), but not for long as she usually use life drain and whatever destructo staff she chooses (fire, ice, shock). As for body armor, she hasn't worn any yet, in my experience.
I gave her the Circlet of Destruction and Ring of Destruction, but haven't really noticed an upswing in power.