Being a vampire is a pretty good deal — unless you get turned as a child…
I don’t actually think the sentence should trail off, but I left it that way so I could show the right way to do it: with three dots, not two.
(The views expressed in these “writing glitch” examples do not necessarily reflect the views of Mercenary Proofreader. In fact, Mercenary Proofreader has had a bad attitude about vampires and vampire fiction ever since a small gang of vampire-wannabes tried to murder his twin two decades ago, so he definitely doesn’t think being a vampire would be a good deal regardless of when one was turned.)
Also, the dash can be replaced with a comma if you don’t want to draw extra attention to the latter part of the sentence. (It isn’t an abrupt change of idea, which is what a dash would indicate.) Another way to write the sentence: Being a vampire is a pretty good deal… unless you get turned as a child. It all depends on where (or if) you want to have a pause/trailing off.