Can the vampires talk?
Though they never actually talk in the movie proper, I say we have to guess that the vampires can talk to each other.
Murcheson says they have to act on the idea that the creatures are working to a coordinated plan. That means they have to communicate with each other, otherwise there is no plan. And the vampires do appear to be actually be working towards some logical end. They attack the power plant because it will apparently be helpful to catch their victims in the dark.
And in the movie itself, the vampires are capable of making sounds. They grunt and some scream when shot in the head.