Why do vultures "waste time" by circling their dead prey instead of swooping in and eating immediately?

A vulture attacks weak prey. They hunt by sight, so they often "watch over" the prey until it is ready, by their standards. Although many vultures find prey using sight, others focus on smell to find their prey.
Sometimes vultures wait for a larger feeder to attack the meat first, so that the meat is perfect for eating.
It is like a human with steak. A man can wait over a grill for his steak to be done and although it is perfectly edible for some, the steak it not yet how he likes it, so he waits.

Contrary to popular belief, vultures do not circle over dead or dying animals. They soar on thermals of warm, rising air. This allows them to best conserve their energy in flight. After rising on the thermal, they glide as far as possible before they need to gain altitude again.

You will certainly see vultures in the air over a carcass, but their descent is rapid. They will not just hang out in the air looking at it... They want to eat it.

http://vulturesociety.homestead.com/Attract.html
In parts of Asia and Africa, vultures have become very brave and comfortable in the presence of humans. In these areas, they sit around rooftops, markets,and garbage dumps, and have much the same presence among the population as the gulls have among us. In such areas, they will follow carts full of food or garbage.

When you see a vulture circling over that hunk of 'possum tartare on the Interstate, it could be trying to sniff out whether the carcass is still fresh enough to eat. "Rotten enough to gag a vulture" is more than just a saying. Contrary to popular belief, vultures like their food as fresh as they can get it.

http://www.straightdope.com/columns/read/1923/why-do-vultures-circle-dead-stuff