Sunday nights on HBO, once the home of mobsters, will welcome vampires this fall. “True Blood,” based on the “Southern Vampire Mysteries” books by Charlaine Harris, looks like it could give the cable network some much-needed buzz since “The Sopranos” ended. The show is set in a world where vampires have come out into the open, thanks to the development of synthetic blood from Japan that allows them to survive without feeding on humans. It stars Anna Paquin (Rogue in “X-Men”) as Sookie Stackhouse, a mind-reading waitress who’s drawn to a vampire (Stephen Moyer, “The Starter Wife”) who arrives in her small Louisiana town. Alan Ball (“Six Feet Under”) is at the helm. HBO is milking the show with ads for Tru Blood, “a synthetic blood nourishment beverage.” Billboard ads with snappy slogans like “All Flavor, No Bite” and “Real Friends Don’t Let Friends Drink Friends,” are popping up all around the United States.