The monster from the 2012 comic con trailer is an unused concept. It was never a full creature designed to be in the movie. That trailer was only a "proof of concept" trailer created by Edwards to prove to Legendary he was on the right track with his vision of the film.
The monster in that trailer is not in the movie, nor does it actually have a fully realized final form. It was only used in the 2012 teaser as a way for Gareth to say "this is a mood piece with this kind of an aftermath. Giant monster burning in a ruined city. Godzilla is coming."
It's just a nameless, unused concept. Everyone calling it Vishnu does so purely because that was the name in the Oppenheimer speech when the creature was on screen. But that monster does not actually have a name, nor a final form. It's an unused concept that will never see the light of day beyond the 2012 trailer.