Where to start...
First off, vampirism and the related fringe offshoots are for the most part harmless. They are generally an immature, poorly thought-out response from a social misfit to wanting to belong to something, because our society and our parents fail to teach kids about their own inherent self-worth and about individualism. Kids feel a need to belong to something special, even the outcasts.
Add the Ann Rice books. My wife loves these, BTW, I never could get into them. The whole homosexuality issue is a common theme in much female-written fantasy and SF, and has more to do with underlying feminist and anti-male bias on the part of the authors, which they typically act out by emasculating their male protagonists through bi- or homosexuality. Popular fiction gives the kids source material for their fantasy world, where they can become smarter than the nerds, physically more powerful than the jocks, and more socially powerful than the "in" crowd through the mythical properties of the vampire. Plus they get to wear a lot of black, and what says more about how much contempt you hold for everyone else than dressing like a full-time dungeon employee?
For the most part, they are harmless.
As far as the "gang" element goes, I have been to numerous gang investigator conferences (and middle America does have a need for Gang Intel folks, but that is another post), and Statanism is invariably one of the breakout topics (I usually go to the raid planning or firearms courses, but I've gotten stuck with occult stuff before). Gang units tend to pick up occult issues because no one else in local law enforcement knows about this stuff, or wants to deal with it. For the most part, this stuff is absolutely harmless and does not broach the level of "criminal predicate" where law enforcement has to be involved at all. Many suspect "occult" incidents turn out to be something else entirely, like a series of local pet mutilations we had a while back. After lots of loud outcry and negative press, and hundreds of man-hours spent (many of which involved NVG and AR equipped officers prowling the woods with orders to shoot the first coyote they found and declare victory), officers captured the culprit-an escape leopard (someone's illegal pet), complete with some undigested pet collars in his gut (don't worry, the leopard is doing fine at a wildlife refuge).
There have been numerous documented cases of occult murders and other violent acts, but for the most part this stuff is pretty rare. The folks who make their money off this are just making money. Ever read a REAL interview with Marilyn Manson? That guy is pretty dang smart, and I don't believe for a second that he actually believes in most of the stuff he puts out in his media material; he is just making money.
As far as condemning any religion, we have a First Amendment in this country, and religions that do not advocate criminal acts have every right to exist, no matter how screwy they are to the rest of us. That being said, when they cross the line, lock 'em up. I knew a "coven" of Wiccans in the Army who got a lot of time at Leavenworth, and I got some pretty big laughs out of that one because they were all incompetent morons who would have been fragged if we ever went to a real shooting war.