Vampires should exist because this means werewolves, unicorns, wizards, etc. could all be real. Also we could use blood packets and willing donors to help feed vampires. Also vampires are awesome (with the exception of Twilight vampires). Vampires are cool, and society needs vampires because vampires can make you awesome.
If they did exist, people would be in serious danger from vampire attacks and could have the dangerous potential to turn humans into vampires with their bites. Vampires are said to be soulless evil blood sucking immortal creatures of the night that craves human blood, so they shouldn't be real.