Do you have vision of the actor and vision of the actor as a vampire? If so, you could just do a simple transition from one to the other, depending on the style of your project.
If you wanted to, you could get a nice morphing plug-in from someone like CHV [
www.chv-plugins.com] and do a lot more work but get a much more impressive result.
If you don't have vision of the actor as a vampire, but want to create one, you could always take a freeze of the actors face into photoshop and build a nice set of fangs in photoshop. Reimport this and morph from actors face to actors -face-with-fangs. This will only be a freeze frame though, unless you are the kind of masochist who is happy to sit animating frames in photoshop. (guilty)
After this there are a bunch of options that require a lot more money, time and expertise.