I doubt it, unlike Mab, I don't think he was ever human, if he was it was so long ago it no longer exists.
Yes, he did, but I don't think having a soul and being nothing but soul are the same thing.. Related maybe, but not the same, lets not forget that this really isn't an area of Bob's expertise. Harry himself said that back in Cold Days when he used soul fire to break the bonds Mother Winter put on him.
I doubt he Vadderung began as a mortal, but he has become mortal before. Even TWG did. It's a complicated question.
Uriel said that mortal's ARE a soul, they HAVE a body. So it's clearly not just one thing. The body clearly shapes the experience of the soul...yet the soul is the more important part. Harry is often wrong about some of these big things (like his conversation with Uriel)...his exposition in that moment fight Mother Winter's will might be an incomplete epiphany.
Having nothing but soul is different clearly from having a mortal body and a soul...but clearly there are beings like Bob that are not soul as well, and physical creatures like ants that are bodies without souls (maybe..?) and beings like Red Court vampires that seemingly don't have souls. I wouldn't be so quick to rule out Vadderung as having or being made of soul. It's just energy after all and gods are energy beings.
Then how does he use Soulfire? Because he does.
Angels use Soulfire. Being mortal isn't a requirement. In fact it seems to be highly unusual given the reactions of divine beings.