I personally don't like the idea of the Wamphyri being so damn unstoppable while the humans are so weak and pathetic. Really, beside the leprosy, the Wamphyri have to have some sort of pay-back for their powers. I know the leech is supposed to remove their conscious and regret, but that is hardly something most people would miss. Also, I fall under the belief that something which does not die or have been built cannot possible have been alive or be destroyed. Argue all you want, but to me the wamphyri are most definately alive and kicking. Are the wampyri as immortal as they say to be or do they just have extremely ridiculous life spans that can stretch for millenia?

In order to make an idea work in fiction there are liberties that an author has to be granted in order to tell their story. Real Wamphyri would probably be incinerated through some mechanism or other as we are all technological cultures now but imagine that the leech would go from person to person. In many circumstances you could catch leech and vamp together but in certain situations the leech would simply, and quickly, escape from harm and find the first host. The premise is, of course, the tenacity. In our current world it would probably quick evolve to something that moves very fast and would likely infect animals at a moments notice and moreover split itself so that it would live a long time in case it were to be put down. Tenacity and evolution are an amazing thing.

And don't forget, the Wamphyri are susceptible to other afflictions such as the Black Death or plague that forced Radu into his resin tomb. When you think about it, they're flesh and blood, but possess extreme strength and healing powers. They are just as easily injured by fire or blade as humans and their inability to be exposed to sunlight is another huge weakness. So they do have their weaknesses and are definately not unstoppable.