Eh, minor details that can be worked out. A human being alone can go 3 weeks without food. Ample time to find suitable candidates I would say given that at the time London had a population of several million people. In regards to vampires discerning people I seem to recall a few tidbits of lore of vampires being able to read minds. Even if they can't, they got at least 3 weeks to find an appropriate target to kill based on just investigation alone.