I wouldn't use the Humanity houserule. The whole point of Humanity (and the Morality system in general) is the fight against baser, selfish urges in a world full of selfish individuals.
Even moreso for Vampires (and to a lesser extent Werewolves), as they've literally become monsters and have to really fight to see normal humans as people instead of, say, cattle. Being unable to lose Humanity rather defeats the point.
So what if it makes Vampires 'weaker' when they gain more Humanity (this is debateable, anyway)? The World of Darkness is not Oerth. The whole point is the story you're collectively telling, not who has the highest damage modifier.