[@AtomicNut]
Most simply bite their victims, draining their blood while transplanting it with vampiric blood. This transplant will then start the transformation process, which usually takes about an hour to complete.

Of course, other strains may do it differently. And certain vampire strains have been known to use small amounts of vampire blood to turn humans into their 'thralls'. Other vampires possess ways to create hybrid servants, and in one case a vampire created a ghoul-ifying virus.

So yeah, there's definitely the possibility of ghouls.