why is the vampire girl's face split?

Nah I think that was by design, and she was a succubus I believe.

It's vaginal symbolism; look up the concept of (I think it's called) vagis dentata, but yeah she's a succubus.

i assumed it was because the naked guy hit her with the shower rod.

Look at the area where her face splits before transformation:

http://imageserver.moviepilot.com/-6449308f-cf68-4183-9bf4-d822f91c430 2.jpeg?width=620&height=372

That's simply what happens when she reveals her true form.