Does the victim have to drink the vampire's blood to be transformed or something? I can't remember. Clearly it's not sustainable if every person whose blood is drunk becomes a vampire, because they'll all be competing for a dwindling food source.
EDIT: Also, good to see Merlin well enough to draw a comic again.
Yeah I think (in Dracula) it involved drinking the vampire's blood too, and also possibly being drained of all of your own blood, though it seemed something that took place over a long period of time and involved a lot of entrancement... and involved a lot of unsubtle turning towards SIN (gasp not our pure ladies).
Figure something quicker but not entirely dissimilar works here, though specifics? Might need to wait for more vampire comics >:3