Holly wore a cross to ward them off

Normally, one would wear a cross to ward off vampires. Holly, however, is wearing one in the hopes that the police, seeing that she's a christian, won't search her quite as vigorously as they might a non-believer. She'll need to stumble into a couple more masses before she finds the genuine religion she has at the end of the album. You get another possible vampire reference later in the verse: "They mostly lay low in the light," this time in reference to the sketchy denizens of the Banging Camp.