He used to be human, well, maybe he never was. The point is, he never was really powerful, and not a treat to any creature with some power. His true power laid in his manipulative powers. After what happened in the tavern, some explanation might be useful.

Now, the point with Vince is, although he was human, his soul was tainted with evil. That isn't much of a problem, almost every creature has this tainting, it's the evil that lives in everyone. Just, in Vince's case, the tainting of his soul was so great it actually affected his body structure. Because of this, every single part of his body has a taint, down to the very molecule. It didn't really affect him, it was impossible to find with human devices, and supernatural creatures might have felt it if they payed close attention. The point is, it didn't effect him, he didn't even know it was there himself.

The recent events around him somehow activated this power. The electricity caused the taint to grow and mutate, completely engulfing his body. It caused his body to melt, and slowly turn into a pool of demonic slime. Normally, his soul would've been engulfed too, practically meaning he would die. This didn't happen, only because his soul has been tainted so much, it actually consisted out of it. This made it possible to exist within the slime. Only to take control of it, and revive his old body, only

The fact that he consist purely of the slime now, means that he can in fact choose any form he wants. The slime just gathers around his soul, forming into a body. this time purely consisting of the demonic slime, in fact, making him a Demon.

While this body is comparable to a shell, He has the possibility sort of ''merge'' with the slime, resulting in his demon form. This form is in fact the manifestation of his soul, and his most powerful form.