knowing your soul is one thing, protecting it is another all together, he has shown no abilities of yet to show he can protect his soul from Raziel, and being a soul for all those years he was helpless inside the frozen thrown, he was starting to die or would have died if Arthas didnt free him in time thanks ot the fracture in the frozen thrown, so id say his soul is certainly vulnerable. Kain is not unfamiliar with the soul, since the whole of LOK more or less reolves around spirits and such but it doesnt save him from being reaved