Your vampires are not undead? They don't just procreate by turning humans?

@Vogon Poet No, they are not undead. Outside of the mountain city, there are very few pure-bloods, from which come a small # of the few newborns that actually live in the city. The vampires in the city look down on turning humans and it is very rare for the act to be condoned for these vampires. In their eyes, the only worthy (or ‘clean’) vampires are like them, pure-bloods from long lines of pure-bloods. Kind of like the prejudice in the Harry Potter series.

Only one birth per female isn’t enough to replace the population. They’d go extinct. No species will have generated a large population if they can’t breed more than replacement rate. They wouldn’t have evolved in the first place. So whatever the reason for low births, it has to be new, probably environmental.

@SRM The answer to that problem is in the last paragraph and hidden behind some jargon for the rest of the post. This setting apparently has vampires that can reproduce in two ways: the human way and the vampire way. Thus the population can grow the vampire way, and be maintained the human way. (That still requires a birth rate of 2 per female, but that's a simple off-by-one error.)

@SRM, pureblood implies that they're purposely inbreeding, that may explain this decline.

Whats the difference between pureblood and newborns ? I didnt quite understand.

@user28434 No. Pureblood is defined in the original question simply as those who are born vampires. Newborns are humans that are turned. It's not a Harry Potter thing.

@GamerGypps Purebloods are born vampires; newborns are humans turned into vampires.

@SRM, yes, but they lived "for thousands of years" in small pureblood community. And "newborns" (new genes for their shallow gene pool) are rare and few. Inbreeding it is.

@SRM - Well, if they also turn enough to keep ahead of replacement rate, it could work.

Because they suck?

@user28434 If they cannot reproduce more than 2 per female, they didn't live for a long time in a small community because they never became a community. The first Eve and Adam only had one kid, and that kid married no one and the species died. Even with speciation happening later and gradually weaning from the source population (the vampire precursors), you still have a point where vampires quickly vanish. Stable population requires at least two births per female, and that's assuming no one ever dies without reproducing (unlikely)!

@SRM By referencing Harry Potter, I was simply trying to express that they are basically ‘racist’ against newborns

@P.M.B. Yes, I understood your point. And I noted that that theory appears to be rejected by the original question text.

@user28434 Do you know how small a population demands inbreeding? Possibly based on the conditions I proposed? As I said, the city is vast and I never said that the population was small, only the mountain range that the city lies beneath.

@P.M.B., it may be big city. But if this world works like our world, Predator-Prey Mass Ratio requires that there should be way more prey than predators. So most of the city should be occupied by the "human cattle". And blood diet may be so low caloric, so it would be like 95%+ of the city for the human. So unless there's nutritional magic involved, your pureblood vampire population would be very tiny.

@user28434 I’m downsizing on several vampire tropes, including the idea that they kill a human every time they drink blood and how much they need to consume on a daily basis. Basically, they don’t use them up like tissue boxes. I was intending for there to be more humans than vampires, but only by enough that one human could be ‘assigned’ to each vampire (like a slave), though the nobility (the very smallest class in the system) might have a few extra for each family.

@P.M.B., even if they don't kill the "prey" problem stays, because it's mass ratio. If they don't kill, they consume less nutrient per human, they consume less nutrient per human, they need more humans for each to feed. It's almost like switching from eating beef and chicken to eating milk & eggs. Yes, it's more renewable and "humane"(if that's the reason vampires don't kill), but you need a lot more milk and eggs to have same level value. And more dedicated "pastures". It would be a lot easier to make all pureblood vampires nobles of the society, and humans(&"newborns") as commoners.