Personaly, I never really liked the vampires, but I need to confess that EA really surpassed itself with this expansion. Vampires ain’t hungry but thirsty. The need of honger is now called thirsty. Vampires won’t attack Sims, but with enough contact they can persuade Sims to give plasma, blood. As a vampire you have certain Gifts which will make the socializing a bit easier, they can read someones mind and figures someone’s traits out a bit quicker. You can also learn skills a lot faster but only at night. A vampire can also go to hospital and get plasma, but you can only go in at night, at day time you cannot go outside of course. The change they get blisters all over their skin is pretty big. A cool feature is that a vegetarian Sim drinks fruit plasma. This fruit is a great solution which replaces the real plasma (blood) that the not vegetarian Sim drinks.
Vampires can have children, if you have one vampire and a normal Sim, the change you get a vampire child is 50%. A vampire child will have the positive qualities (like better visions) and won’t have the bad qualities, they will get these when they become teenagers (like getting blisters from the sun). If you don’t want to be a vampire any more, then you can go to the scientific lab and get a potion. If you do not want to play with vampires, you can just ignore them and after a while it feels like they aren’t there.