Ok so this has been bugging me a while and I keep seeing people talk about the vasto lords and why we still haven't had a character pointed at and called one directly. Normally I just sit back and read but this has been swirling around in my head a while and I honestly think we've blown right past them and they are no longer important.
If you check hierarchy, the hollow go like this:
- Vasto Lorde
There's a quote somewhere that says that a handful of Vasto Lorde could easily destroy Soul Society. Since no one in Soul Society had ever really seen one I think it was just an exaggeration or a bluff of some sort. The Vasto Lord were supposed to be smaller, human like hollows that were of captain class or higher.
There's more info on the wiki:
I know the manga and the anime don't always line up exactly but if you watch Bleach anime episode 284 you will see that Halibel is referred to as a Vasto Lorde twice in the episode(Sorry I don't remember the times). Compared to the Adjuchas that are shown throughout the episode, you'll notice that only her and Barraggan have a "human like" appearance. I really think that these two characters were Vasto Lorde back before Aizen got a hold of them.
When Aizen showed up in Hueco Mundo he started making the Arrancar. The Arrancar are hollows that have had their masks removed or partially removed, a result of the process Aizen used to make the hollow stronger. The stronger the hollow are before the Arrancar process, the greater the result and the more human like they appear. After the process, Baraggan is the #2 Espada, and Halibel is the #3 Espada.
At the end of the anime episode 284 Aizen says that the combined strength of all the Espada-Arrancar is less than that of his own, and he calls the Visored a failed attempt at making Arrancar.
If you put it all together I think it's safe to assume that the Arrancar form created by Aizen is more powerful than that of the Vasto Lord. Also I think that because Aizen was the one doing all the research on hollowfication, he probably was the one spreading the rumors or bluffs on how powerful the Vasto Lord actually were.
Edit: watch the episode here if you want.