I get really sick of authors who happen to be Christian getting bashed for whatever they write--by non-christians as well as by other Christians.
If they write Christian books, some people say they don't have the guts to write mainstream books. Other Christians criticize them for which subjects they choose to write about or how they handle them. Now non-christians (forgive me if I'm getting the wrong idea about your beliefs) are saying Christians aren't qualified to write about vampires?
I am a Christian, and my faith influences what I write about & how I do it, as I'm sure everyone's worldview does. I've read books by some Christian authors who've taken their stories to places I don't think I'd go. But I think it's dangerous to say certain topics or genres are strictly off-limits to Christians. To me, the ethics of it is all in how they're handled.
In the case of Rice's Vampire books, the complaint seems pretty silly. It's my understanding that Rice only recently rededicated herself to her faith. She did not sit down to write books with a Christian message at the time those books were written.
Aside from that, you may be enjoying more books with Christian themes than you realize. If you really think about it, if you threw out all the books with themes that could somehow be related to Christianity, there would be very little left. Redemption, good versus evil, self-sacrifice, the hero who saves the day, spiritual or character growth, "messiantic" concepts such as a person being special or chosen to do something great, forgiveness--I could go on and on.
If you can't enjoy a book because you suspect Christianity may have influenced its author in some way, what on earth are you going to read?
I can uderstand not wanting to get whacked over the head with a religious message you don't want to hear, but come on. Rice didn't do that in her vampire books. The fact that you liked them until you got the news that she's a Christian proves that.
All of Western civilization has been influenced by Christianity. That includes literature. It's unavoidable.