In most vampire fiction, vampires are lavishly wealthy, often living in expensive homes and wearing fine clothing.
TV Trope description[]
In most vampire fiction, vampires tend to be filthy rich or at least comfortably loaded. It's presumed that over the years they've managed to save/squirrel away money so that they can have a comfortable lifestyle, plenty of security so nobody comes in and stakes them in their sleep, and a fabulous wardrobe.
Some certainly look like the Rich Idiot with No Day Job, others may be Non-Idle Rich and manage to be financially enterprising. Generally justified in that the vamp is hundreds of years old, and thus has had plenty of time to accrue his wealth (and invest it via The Slow Path version of a Compound Interest Time Travel Gambit). Being technically dead also saves several bills, as there's no need to buy food, water, health insurance, you get the idea.
Tropologically, it's probably to do with vampires being decadent aristocrats going back to Count Dracula. Both tropes play on the symbolic connection between literal blood-sucking and the parasitic way of life of an (assumed) real-life idle class which does not support itself by its own efforts, but by exploiting other people. Vampires made in recent years, however, may not fall into this trope due to a Subverted Trope or lack of savings, and may end up having to work nights at the Quik-E-Mart in order to pay for a dark enough apartment to sleep in.