Because I wanted a tribal component, I needed the Innistrad Vampires in two colors. Black was a gimme, but the second color proved harder. I could make a case for Vampires in any color other than Green, so I decided to leave them until I made other tribal decisions.
Zombies ended up in Blue and Black. Blue was chosen because we wanted Frankenstein’s monster-style zombies as well as traditional Zombies. Werewolves ended up in Green & Red because that was where they made flavor sense outside Black (each color only got two tribes).
Blue was out because two tribes couldn’t be Blue & Black. Green was out because Green Vampires didn’t make sense (yes, I’m sure people in the comments will explain why they would be awesome). White was out because I wanted all the White creatures to be benevolent and that didn’t make sense for Vampires.
That left Black & Red for the Vampires. I needed a more aggressive tribe and blood-hungry impulsive Vampires sounded cool so that’s how they ended up in Red.
EDIT: Oops, You said Zendikar not Innistrad.
Zendikar Vampires were when we finally decided to stop using Vampires as an iconic race and start use them as a characteristic one (instead of infrequently and splashy at rare, frequently and less splashy at common and uncommon).
Zendikar Vampires will return in Battle for Zendikar.