Humour is invoking some fear that suddenly gets removed by the punchline (failure to remove the fear makes the joke offensive).

So by stating the process which is strongly associated with vampires thus "vampire" neuron activated, which in turn activates names that are associated with vampires such as "dracula" neuron (the neuron has to be learned though so the audience needs to have heard about dracula before else there is no such neuron and the joke will fail).

So with the dracula word sent into the consciousness due to "dracula" neuron activation, Dr. Acula will add enough activation strength to the imagined word via the similarities that the "dracula" neuron gets activated the strongest, more than any other vampire name so there is confidence that it means dracula.