How are Vasto Lorde different from the Espada? There are a lot of threads on the internet that have confusing/contradicting answers, but how are Vasto Lorde different from the Espada?
Vasto Lorde are the top of the Hollow species. However, they are Hollow, not Arrancar.

Arrancars are Hollow which, using the power of the Hogyoku, removed the boundry between Hollow and Shinigami, and achieved a greater power, usually by removing their masks and sealing most of their power into a sword.

Espada, are by definition the top ten Arrancar. They may or many not be Vasto Lorde.

It was heavily implied that of all the Espada, only the top four (Ulquiorra, Halibel, Barragan and Stark) were Vasto Lorde before they became Arrancars (and subsequently Espada).