Board Thread:General Discussion/@comment-31283382-20131101162239/@comment-1268838-20131101182854

Vasto Lorde is the highest level of Menos evolution; Arrancar is a state of being that any Hollow, evens ones that are not Menos, can become.

Technically we have never seen exactly how a Vasto Lorde comes into being. We know (or at least it was implied) that Grimmjow failed to do so. I think that the simplest explanation is the one that Shawlong offered; unlike the case with Adjuchas, only a select few Hollows have the potential to become Vasto Lordes. At that point, it's not just a matter of eating more Hollows.

Also, we only know for sure that at least half of the Espada are Vasto Lorde-level. It is speculative, but I think that the "rank 4 and above cannot release under the dome" rule gives us a good idea of who is what.