Well, if we first accept that the vampiric condition is passed on through vampire blood, and then accept that the condition stops the aging process (hence the wonderful condition of the old man vampire who turns out to be 121, and his apparent granddaughter who is 2500, but I digress), then we have a problem. Even if a female vampire could still ovulate, and an egg becomes fertilized, and if the egg was based around her original dna rather than any changes that might have occured, then eventually the embryo is going to settle into the womb wall, and become a foetus.
And how does a foetus get it's necessary nutrients? Through it's mothers blood. (Three billion women scream "That's a gross oversimplification!" - well, yeah. The point stands though) So the foetus is fed vampire blood, and - there and then - becomes a vampire. Equally, while it doesn't have to worry about sunlight too much, and doesn't have to go far to feed, it also means the aging process stops. The foetus does not grow any further whatsoever, and remains in its present state. What is more, because this state would occur very early on in the pregnancy, the mother would never go into labour. The foetus would then spend the rest of it's mother's unlife parasitically taking her blood, while never maturing - unless it was removed. Or, of course, unless it chews its own way out. What a lovely thought that is!