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).
Hallows have 3 ranks apart from their basic form : 1st rank Gillian also called menos Grande 2nd rank Adjuchas 3rd rank Vasto Lorde these hollows keep their hallow forms.
Now Hallows who attained powers like a soul reaper are called Arrancar (which means to rip/tear off) and do not have the hollow defining skull mask and look human like with only a fragment of their mask attached to their head. Espada are simply the top ten strongest Arrancar in Sousuke Aizen' s army of Arrancar. Hope this was helpful
Vasto Lorde are extremely powerful hollows. Hitsuguya suggested that if Aizen were to get ten Arrancars on Vasto Lorde level, the Soul Society would be doomed. For
Vasto lorde are weak ... Nnoitra(when he was 8 in espada ) was sent to recruit vasto lorde but he killed him
user4640 is mistelling the information. In episode 202, Nnoitra was talking to Nel, who asked him why he destroyed an entire colony of hollows. He said he did it because he was not told not to, as well he was looking for Vasto Lorde. The exact quote is,
If I could kill them(Hollows) off that easily, then they couldn't be Vasto Lorde.
Espada are extremely powerful Hollows made into Arrancars. Arrancars come from the Hogyoku, where the Vasto Lorde cannot come from being an Arrancar. Arrancars lose the evolutionary step because they are in between Soul reaper and hollows. Only hollows that do not become Arrancars can be become vasto Lordes. However, Vasto Lordes can become Arrancars.