Where to start............ Ok, crazy before sharing an ad-on... make sure all the essential part of the add on are aparent...... Second, a vampire isnt like a super man player... make the vampire like a normal player but a little faster..... Now this part is optional... make a vampire model for the player type. There you go. < this is jitank btw >
Edit: I forgot to add make the player either drain all the blood and kill the player or infect the player with vamporism.... that would be vary neat