Vampires are also rare because of vampire purges, a type of worldgen event introduced a few versions ago. Whereas werebeasts can often live forever if no one bothers them, vampires are more actively hunted and destroyed - and since vampires lack the ability to personally create new vampires (unlike werebeasts) the supply is always pretty low.
Having people become vampires via secret won't make them build towers. To build towers a creature needs to be able to animate corpses, which vampires can't do.
However, I think such a vampire secret would be quite rare without towers. Secret-knowers won't teach apprentices unless they live at a tower, and they are much, much more likely to write books (including books about the secret) at a tower (they can write books without a tower, but only incidentally). So I'm not sure adding a vampire secret would strongly increase the likelihood of vampires.
You might consider using advanced world generation to increase the number of vampire curse types relative to werebeast curses - then, whenever a dwarf is cursed, they'd be much more likely to become a vampire than a were, so there should be more vampires on average in such a world.
You could make god-cursed vampires turn into a monster - "ancient vampire" or something - that acts like a werebeast, going on rampages and biting people, and the bite would turn people into normal vampires. That should give you a continuously-increasing supply of vampires throughout worldgen.
Or... you could give vampires an interaction that straight-up turns another dwarf into a vampire. That's a thing vampires do, right? Maybe let them resurrect corpses as vampires, like after they drain someone, if you want to be truer to form. Not sure how well that would work, though.