What personality does a vampire have?

A vampire is merely the re-animation of the person he was before he died. So a vampire will have the same (or nearly so) personality that he had while he was living. Now you are what you eat and you are what your situation requires you to be. So you also have to factor in his environment, and his company and his needs: all this will have an impact on how his personality is expressed.

It should be noted that 'becoming a vampire' is a delicate process and if it is not carried out carefully the resulting vampire can be extremely malformed. This, coupled with the revulsion that most people have to vampires, often results in the new vampire destroying himself. Therefore it is mostly those people who already have a high ego that chose to retain their existence as a vampire and consequently make up the personalities of most the vampires.