One of the main questions in Bleach is whether or not the Espada are actually Vasto Lordes. The technical answer is no; as being an Espada does not consequencly make a hollow Vasto Lordes. This is confirmed by Aaroniero Arruruerie as he tells Rukia that he is the only Gillian-class hollow within the ranks of the Espada. This confirms not only that being an Espada does not make you a Vasto Lordes, but that most, if not none of the Espada are actually Vasto Lordes rank. If most of Espada were Vasto Lordes, than the chance a Gillian would make Espada above an Adjuchas is almost zero.
Then there is Toushirou Hitsugaya's explaination of a Vasto Lordes. He states a Vasto Lordes is about the same size as a human. Contrary to popular belief however, he never says a Vasto Lordes apperance is also similar to that of a human- which many have used to rule out Ulquiorra and Nnoitra from possible Vasto Lordes as they both have very animal-like apperances in their released forms. When taking out that a Vasto Lordes is similar to humans in apperance as a classification, the number of Espada that could classify as Vasto Lordes increases to being all of the Espada except Yammy, Arroneiro, and Szayel.
Then Hitsugaya also says a Vasto Lordes exceeds the strength of a captain. This is too say if an average captain level shinigami fought an average level Vasto Lordes, the captain would loss. Using this standard, only Grimmjow and Zommari can be respecively removed from the list of possibles, as Ichigo is able to defeat Grimmjow on his own and Byakuya- Zommari. The reason I say this is because although Ichigo has Bankai and stronger than Captain level Spiritual Pressure, his overall skill and strength is only that of a base captain level shinigami. This is proven from his first fight with Byakuya where he was only able to be victorious because his hollow intervened when his body had reached its limit mid-battle, where otherwise he would have lost. Using this, Byakuya is probably one of the best example of an 'average captain' in the Gotei 13 and he was able to defeat Zommari with almost no none-self-sustained injuries to him.
Although Nnoitra is defeated by Kenpachi on his own, he cannot be removed because Kenpachi is under no circumstance an 'average captain.' And since Ichigo was only just able to beat Grimmjow at 'average captain' level, it is most likely that Nnoitra could deafeat Ichigo even if he were at full strength. Also, Although Stark is for the most part defeated solely by Kyoraku, Kyoraku managed to defeat him while remaining in Shikai, making him in no way an 'average captain'
Nnoitra is defeated by an above average captain; Ulquiorra is deafeated by Ichigo only after he undergoes a full hollification and his hollow is the one actually fighting; Harribel is only taken down by Aizen, whereas if she had continued fighting Hitsugaya, who like Byakuya is one of the better examples of an 'average captain', she might have ended up winning the fight; Barragan proves numerous times that he is more than a match for Soi Fon, who is another of the better examples of an 'average captain'; and Stark is defeated by Kyoraku- an above average captain, while he was still in shikai, although this was only after several interruptions from Ukitake and two of the Vizards, making the possiblity he could have beaten Kyoraku had it as a whole been one-on-one increase if only slightly.
Of these five, the only ones we see the past of from before they became Arrancars were Barragan and Harribel, whose Resurreccion forms are also exceedingly close to- and in Barragan's case identical to- their pre-Arrancar forms. Also of the five, the only ones we actually see come in contact with Aizen prior to becoming an Espada were in fact the top three: Stark, Barragan, and Harribel. The fact they are the only ones we know of that were personally met by Aizen prior to becoming an Espada increases the chance that they are indeed a Vasto Lordes.