I observed the same creatures seeming incapable of speech. As if the hunger had diminished them to their most base instincts. Yet others who had fed, who had been turned recently, could speak with eloquence and understanding.

You are right. It is indeed complicated. Perhaps the vampires simply were unaware some would volunteer to provide sustenance. And perhaps some cared not to even ask. I do wonder if they have a society or hierarchy. Perhaps finding what leadership they have, if it exists, could stop another such attack before it begins.