There is no need to worry about that fortunately. Vampires can sometimes drink from sims autonomously (when they are very angry, too thirsty, or are breaking into your sim's house), but they will never autonomously turn sims, nor will sims ask to be turned.
Vampires may sometimes ask your sim if they want to be turned, but you can refuse it. When you are controlling a vampire with the ability to turn sims, you can go ahead and turn any sim into a vampire without needing to ask first.
However, it is unknown whether vampires in a club gathering in where one of the rules is "Use Vampire Powers" can autonomously turn sims or not. I have to test that out.
Do also keep an eye on patches as I feel like one day Neighborhood Stories will get updated with occults (becoming and getting cured). While it will not affect played sims, you can change it through Manage Households panel.