Compared to the Axeman, which this unit replaces, the Vulture has 1 more point of strength, but loses half of its bonus against Melee units. This means that it actually performs equally well against melee units before promotions (since 5 * 1.5 = 6 * 1.25 = 7.5), but it gains more from promotions and does better against non-melee units.
Also, due to the way combat works in Civilization IV, Axemen actually beat Vultures badly.
The Vulture was not a specific type of troop within the Sumerian army, but rather a symbol. Warfare was a fact of life between the city-states of lower Mesopotamia, but it was during a battle between the King of Lagash and the King of Umma in 2525 BC that something exceptional occurred.
After a resounding victory over the King of Umma, the King of Lagash erected a stele to commemorate the occasion. In commissioning this monument to his victory, the King of Lagash created the first known depiction of war ever created by humanity. The stele's carvings display the troops of Lagash: spearmen arranged in a phalanx formation, with eight men to a row, six rows deep. (As a phalanx arrangement would require strict training and discipline, this suggests that the Sumerians had one of the earliest professional armies.) Elsewhere on the stele, vultures can be seen carrying away the heads of the fallen Umman warriors. As a result, these Sumerian soldiers have become known as the "Vultures of Sumer."