Blood, by its very nature, is an oxidant. As such, it's the exact opposite of an anti-oxidant. This may sound obvious, but thinking through the implications a little deeper makes one realise that there are some serious differences to nutrition going on in a vampire's physiology by comparison to a normal human.
In point of fact, this is a paradox in how vampires and their physiologies work; on the one hand you have creatures with incredible recuperative and healing powers, which implies over-oxidisation and a higher metabolism, but then they live extended life spans, which implies under-oxidisation and a lower metabolism. For the purposes of this question however it is important to note that anti-oxidants are supposed to remove free-radicals from the body, reducing the amount of damage done to it by left over oxygen artefacts generated by normal metabolic processes.
It was thought for some time that anti-oxidants actually improved fertility in women and increased their chances of bringing live babies to term, but recent scientific studies have largely discredited this. But, it's important to note that these scientific studies focused on normal women with normal metabolisms; there were no female vampires with their unique metabolic processes introduced into the studies that have been done on the subject.
As such, we really don't know what drinking blood as a sole source of energy and nutrition is going to do to a female vampire in terms of her fertility or ability to bring a baby to term, but here is a bit of supposition for you;
For one thing, vampires are likely to have an excess of free-radicals in their system because their source of nutrition is designed from the ground up to bind oxygen to itself. As such, without a nutritional source of anti-oxidants to clean up the excess free radicals generated by the metabolism, these are likely to do a lot of damage to the body over time and may well be the reason why your vampires are not eternal.
Secondly, to live as long as they do, vampires, particularly given the existing issue of no anti-oxidants in their diet, may well have a lowered metabolism. This would explain their ability to survive on only small amounts of blood, and their extended lifespan, but would be contrary to their abilities of super speed or accelerated healing. Let's assume for now however that our vampires have a large number of free-radicals in their system AND a lowered metabolism.
The practical upshot of this is that even if vampire fertility was similar to that of humans, the capacity of the body to bring a baby to term is diminished. At best, vampire gestation rates would increase by the same factor as lifespan (say x5, or nearly 4 years) but likely even longer, and the free-radicals mean there's even more chance of random damage to the body in such a way that something can go wrong with the pregnancy. The birth itself would also be precarious, because the amount of energy required to deliver the baby would be massive by comparison to the amount of energy that the mother has available to her for the process. There would be no margin for error.
Vampires with lower metabolisms are essentially working with bodies designed for marathons, whereas delivering a baby requires a sprint; a relatively short, sharp, but massive expenditure of energy to safely bring the baby into the world.
Now, sure; we're already working with dichotomies like accelerated healing and it's possible that the body can switch between these two modes almost at will; increasing metabolism (and thirst) under times when it needs to heal at an accelerated rate and then ratcheting back down once the healing is done. Same would go for the delivery of the baby, but in my view each of these transitions between metabolic rates would incur a cost, meaning that warrior vampires, as well as female pregnant ones, only have so many times they can make that transition and so bringing more than a single baby to term requires an extraordinary constitution, access to additional blood supplies, luck, and probably all three.
Put simply, your female vampires run cool most of the time, need to run hot when bringing a baby to term, and switching between the two carries both a price and a risk that will eventually kill the mother, and probably the baby.