Not OP as in, she can still be stopped. If she had still casting as well, literally nothing could stop her. xD And no, she can cast level 100 spells. It's just that they're a bit weaker than, say, a human necromancer of lv 100. And sure she could tank most hits from normal enemies...but there aren't only 'normal enemies' in this world. :P (Note, even something like big ass orges are 'normal enemies'. >:3 ) 

Armour is armour, cloth is cloth. Sure they give a ton of stats, but in this rp it's just cloth. Magically enhanced cloth...but still!

Vampires in D&D have dr 6/silver, yes. And 10 cold and electricity resistance. But like I said, this is D&D-like, not actual D&D. :) I'm just using D&D rulesets as a basis, then adding some of my own thoughts to it to fit the rp more I guess. That's the wonders of roleplay. You're generally not bound by set rules like in D&D. :P