Well, recently it's been suggested that there were vampires in their own little down-time nests - I think it's Red who says something about this in one of the recent books, or implies it at the very least. The more powerful the vampire, the more s/he had to 'sleep', to cocoon up and try to survive 5K years off 'stored mana', as it were.
But I'm not sure why you're saying that even if there weren't pre-6W vampires that nonhuman carriers existing would be a chicken/egg thing. I mean, clearly the virus came first; an Awakened virus that mutated so that when it infected a humanoid host, it made some vastly radical changes to the host in order to prolong the virus's existence -- some form of genetic immortality being the goal of every living thing that has genetics -- and suddenly you have the first vampire. I mean, think of it instead as an inversion of ebola; ebola comes from somewhere, a viral infection that can be found in nonhuman hosts in deep dark Africa. Once it hits a human, well, it burns through them ... but that virus which we call HMHVV changes them instead of slaying them.
Simply put, the first Infected would not necessarily need to have been 'transformed' by another of their kind; only susceptible to whatever origin-state elements were required. And the various strains can then evolve from the first one.