Doesn't this mean there's no reversing it?
I was confused, too, but here's the full quote:
If you find yourself drawn to these creatures of the night, know that by befriending a vampire you can eventually ask him or her to turn you, or if you happen to mate, there's a chance that you may end up with a vampire baby. Consider yourself warned: There's no going back.
It seems (at least to me) that the "there's no going back" is in reference to vamp babies. So perhaps if your Sim was born "human" and then was transformed into a vampire, he has the option to change back to human if he so wishes; but if he was born a vampire, he remains a vampire for life whether he wants to or not. Sort of like ghosts; if you have a Sim that has died and become a ghost, if you feed them Ambrosia, they become a regular "human" Sim again, but (as far as I understand it, as I've never really played with ghosts before) if a Sim has a ghost baby, that baby will be a ghost throughout their entire lifespan. Maybe?