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.
This has to be the most idiotic question ever posted here. If Vampires existed, one species would be wiped out. Either the Vampires turn everyone else (eventually) into Vampires or humans kill them all to preserve our species. Seriously though, who thought this was a valid question? I've heard people ask if they do or do not exist, but should they? C'mon, do something better with your time.
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.