I'd go with the idea that the (human) populace views vampirehood as a high honor, and aspire to achieve it. As in, the city holds annual competitive festivals where the populace competes to become the next new vampire. For example, winter has a craftsman/art competition, spring has a magic/caster competition, summer has a singing/performance competition, and autumn has a combat/athletics competition.
Winner of a competition is recognized as a paragon of that particular discipline, and welcomed into the vampiric community. The losing competitors are used as food for the vampires. Not necessarily killed, but definitely preyed upon.
The general population carries on with that idea: a good citizen does his job, and does it well, or he or she is cast down and destroyed. No mercy for the weak. The entire concept of philanthropy is alien.
I agree with the suggestion that religion is outlawed. Carrying a holy symbol is punishable by death. Since in D&D a holy symbol can be virtually anything and take on any shape, jewelry and ornamentation of any kind are almost completely unknown.
Most of the city should be made of stone, metal, glass, and other nonflammable materials. Even furniture should be made of stone (niches with pillows), or stuffed fabric/hide. Wood should be as scarce as possible.
The streets should be overhung by thick tarps, and buildings should have chimneys for ventilation but few windows. Interior light is from oil lamps (the city is by the sea, right?) and cheap magic items such as everburning globes. Many areas of the city should be completely inaccessible except via gaseous form.
-z
