After going through several comments in Bleach forums all over the net, for the past 2 years, I've notice the Vasto Lordes are one of the most debatable issue discussed so far. Many strongly assume that there are ten Vasto Lordes, being reserved in Hueco Mundo, waiting for Aizen's command, and none of the ten Espada's are one of them. The reason many refused to believe any of the Espadas are Vasto Lordes, are largely based on two reasons; due to how they are all defeated, and what Hitsugaya Toshirou description of the Vasto Lordes.
First, let go through, again, what is a Vasto Lorde. A Vasto Lorde, is an evolutionary stage of a Hollow. It is the last stage of a Menos evolution, and the strongest of all Menos class. Its appearance is the most humanoid of all hollow classes. A Vasto Lorde power level, is stronger than a captain, and according to Hitsugaya Toshirou, if Aizen possessed 10 of them, he could easily destroy Soul Society.
Now, moving on to the Espadas. Espada consist of 10 powerful Arrancar, a mixture of both created through Hokyoku, and naturally borned Arrancar. Espadas are ranked from 10 to 1, or according to Yammy, from 9 to 0. Higher number being the weakest, lowest being the strongest. According to Ulquiorra, Espada ranked 4 to 1 cannot release their zanpaktou inside Las Noches, as their power release will seriously damage the palace.
So let's analyze the information.
First, Vasto Lordes are humanoid in appearance, even before Shinigamification. Humanoid means human in appearance, having two legs, two arms, no tails, no horns, etc. Yammy, Aaroniero, Szayel Aporro, Zommari, Grimmjaw, Nnoitra, and Neliel are not humanoid in appearance during their ressuccion release, therefore, they are not Vasto Lordes. Ulquiorra is a bit difficult case. Despite being the top 4, and are not allowed to release inside, his ressuccion release, both forms are not quite humanoid. He had wings, talons, and a long tail. He however, was the only Espada to have a second release, a powerful form which may classify him as a Vasto Lorde in terms of "powerful than a captain". Moving on to the top 3. All top 3 Espadas are humanoid in their ressuccion appearance. However only Barragan and Starrk are truly humanoid. Despite Barragan's walking skeleton appearance, he was a complete human skeleton and human sized. That classified him to being humanoid. Starrk's release was also humanoid, as it was a complete human body wearing a comboy outfit. Harribel, although has a humanoid face and human body, has some skeletal pieces attached to her body. Basically how I classify humanoid, is based on their X-ray skeletal form. What looks human in the x-rays, are humanoid to me.
Second, Vasto Lordes being stronger than a captain. Do note, that Hitsugaya was not talking about stronger than the strongest captain, he was talking about stronger than a captain, probably an average captain. So which of the ten Espadas was more powerful than a captain? Zommari, Szayel, and Nnoitra lost to captain. Despite Zommari and Szayel's special abilities, that could allow them to defeat a captain, they were not Vasto Lordes based on the "humanoid appearance factor". Espadas that are clearly more powerful than a captain are the top 4. Ulquiorra, defeated Hollow Masked Ichigo, even before his Segunda Etapa. Hollow Masked Ichigo was at a captain level in power. Harribel is questionable. Her being more powerful than Ulquiorra, should be able to defeat a captain. Her performance during her battle with Hitsugaya was not really "powerful than a captain" material, however. Barragan was clearly more powerful than a captain. Soi Fon didn't stand a chance against him, even after using Ban Kai. Starrk was also stronger than a captain, as he nearly defeated 2 Vaizard captains without any injuries himself.
Now, for the first factor, "humanoid appearance", only Barragan and Starrk was classified as humanoid. Second factor, "powerful than a captain", only Ulquiorra, Harribel, Barragan and Starrk was classified. In total, only Barragan and Starrk classifies as Vasto Lordes.
Let's see what do these two have in common. Clearly, Barragan and Starrk, were the only Hollows, to be recruited personally by Aizen Sousuke. If he recruited them himself, then they are special Hollows indeed, special as in Vasto Lorde.
However, some debated the two can't be Vasto Lordes as Aizen ordered Neliel and Nnoitra to search for Vasto Lordes. So to them, that means Aizen has already went through Barragan and Starrk. If both of them aren't Vasto Lordes, would Aizen troubled himself to recruit them personally?
The Vasto Lorde issue was clearly overrated. Being stronger than a captain is not being stronger than Soul Society itself. People refused to believe there are Vasto Lordes in the Espada because they believe Vasto Lordes are incredibly powerful, not like what they see today. People overestimated the Vasto Lordes power. If they are so powerful, would it make sense for Aizen to sent Neliel, and Nnoitra, a 3rd rank and 8th rank to recruit them? Would those two be powerful enough to persuade the "mighty" Vasto Lorde to join them?
This is just my point of view, not meant to offend anyone. Vasto Lordes exist inside the Espada. It is an evolutionary stage, not a rank. Which is why, the Espada don't boast about it. Evolutionary stage is like race. It would make sense to say "You cannot question me, I'm a colonel!" than "You cannot question me, I'm an Indian!". Purely for example. Sorry for using "Indian". My apologies. It was just an example.