Can Vampires autonomously drink from other sims?

It might have been, but I am not sure in which patch. They are supposed to once they lose control, unless they have the perk that keeps from autonomously attacking Sims.

I don't know the specifics of your situations, though. It could even be people with sustained moodlets from having their plasma drained prior. Despite how little sense it makes, vampires (even out of control vampires) always respect when Sims are anemic as to not cause them harm.