|It's okay. Twilight is often a source of my anger.|
A vampire, by nature, is not a living creature. Regardless of how the exact process goes down. the vampire bites its victim, transfers some of its blood (or if you sparkle, venom), the victim's body dies as the vampire infection takes over and the body metamorphoses a bit, then it reanimates with fangs and night vision and all that cool stuff. Some key points to address here: when your body dies your lungs cease to breathe and your heart to beat.* Of course, there's no explaining higher brain function, but that's not what we're discussing here...
So. No heart beat, eh? Without a heart beat there can not be blood flow, and without blood flow, there is no male erection (because let's face it, it's always a male vampire and a human female). It was in Anne Rice's novel Pandora that I first saw this addressed, and I apologize in advance since it's been so long since I've read it that I have only a vague recollection of the scene, but the main character, Pandora, was begging a vampire to make love to her and went so far as to try to put his penis in her and he was just "sorry... it doesn't work any more." Because it doesn't. If you don't have a beating heart, you can't get an erection, and you therefore can't get someone pregnant. It's absurd.
And say, for shits and giggles, you could get an erection. Why would you have viable sperm? You already have a means of propagating your species that is both tasty to you and almost instantaneous, without the tedium and mess of childbirth and infancy. Think about it: you bite someone, get a little tasty snack in the process, make a buddy. Such efficiency!
So please, let's stop this half-vampire bullshit. It's just ridiculous. And can we stop the plot device that half-vampires are the only ones that can effectively kill vampires? It's been done. A couple of times.
*I'm giving author Kresley Cole a pass here because she actually uses a vampire's sudden beating heart and breathing lungs as a sign that he's found his Bride. It's actually pretty funny because it usually scares the shit out of him because he doesn't know what the sound is. Yeah. Anyway, as a plot device it shows a major vulnerability to his enemies when they can tell he has a pulse again, so while yes, her vampires can have sex once they find their Bride, they still can not impregnate them.