That is a point of vampire physiological debate actually, whether they DO breathe or not. Lestat Says Yes, Therefore it is Truth! … except maybe not. Maybe he thinks he’s breathing because he has to, but he may be doing it out of habit.

I feel like they would need to breathe for the air to go through properly for speech. Otherwise, wouldn’t they sound more like the breathless screeching vampires in 30 Days of Night?

there was a fic in which the vampires considered what would happen if they were in a coffin on a ship and the ship sunk. Would they still be conscious, trapped in their coffin? Presumably they wouldn’t die by drowning… hmmm…?

However that’s just RICEAN vampires. If you wanna know in a more general way, go to askavampirologist ;]