Vasto Lordes are the third level of Menos and the highest level of evolution. They are roughly the same size as humans and are very rare in number. The Vasto Lordes' combat skills are said to be above the captains of the Gotei 13.
Apart from the need to eat even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.
Tōshirō Hitsugaya states that if Aizen has more than ten Arrancar at Vasto Lorde level, the Soul Society would be doomed. (Though he has gathered them, the soul society is still there, and all the vasto lordes Aizen has gathered are dead.)
Aizen states to Gin that he needs to gather enough Vasto Lordes' to complete his ten-member Espada.