Ideas, dearie. You got to grab peoples' attention with sweet ideas.
It doesn't need to be a lot; just a base from which to work from, a foundation of what you'd like your RP to be. What sort of vampires are you thinking of rocking with? Hideous, Nosferatu-esque monsters? Seductive predators? Sparkly emo-things? [size=-5](please for the love of all that is good not the last one)[/size] Same goes for the zombies; are we talking rage-addled nutters, ala 28 Days Later, or your more traditional, slow-moving Romero rotter?
Giving a brief description of your setting or where you'd like the plot to go can help snag interest, too; will this be set in a post-apocalyptic waste, or in small-town USA? How do you want to link the vamps and the zombies, if at all?
All these things (and more; pretty pictures tell a good few words, if clichéd lines are to be believed) will help to generate interest for your RP. It's all about getting peoples' attention, making them want to get involved.