What's this graveyard I found?
It's simply a memorial spot for well-liked players who died (the players, not the vampires). The only effect of the graveyard is that you can use flowers there. Many players will appreciate if vampires are polite when there, out of respect for those who cared about the deceased.
There are other memorial spots scattered throughout the city, for which the same things apply.
Some players take it upon themselves to exact retribution upon anyone causing harm in the vicinity of the various memorials (including the surrounding squares), though there is no game rule against it.