Grimmjow was confirmed in the manga to be an Adjuchas from his backstory shown in chapter 284.
As far as I know, Harribel was the only Arrancar and Espada who was confirmed to be a Vasto Lorde.
Well, Ulquiorra was very strong, so it's not a surprise.
Maybe Vasto Lorde Arrancar are the reason that Espada no. 4 and higher cannot release their swords within Las Noches.
Grimmjow seemed to be close, since he was very small for an Adjuchas. It would be interesting if Kubo provided the necessary qualifications for becoming a Vasto Lorde, and whether or not it was more than simply eating enough or being blessed with that much spiritual pressure.
I think a vesto lordes should be human shape in appearance. also in my opinion, espadas from 1-5 were vesto lordes since with their transformations they were able to retain back their human appearance unlike the other espadas when they transform. thats why I think Mel, at the time she transformed back into her adult self, was considerably weaker than espada 5 while still stronger than grimmjow. again in my opinion, the reason why she was stronger then him years gone by, was because she had more experience thus was able to beat him every time using that. remember guys, experience is the key to victory in every battle. dont matter how stronger u are once u use ur experience u will win. however ichigo is the only exception since Aizen was the one who made him win those battles or other wised he would have lost. look at it guys, how can some 1 with no experience in battles beat opponents with near 100 year experience? Plot shielding.
On another note, as it relates to grimmjow, if u look at harribel's flash back to when she was an vestolordes, all her fraccions were adjuchas some of which was pretty small. so he was not the exception there can be small adjuchas.
If we take Hitsugaya explanation to the letter then we had four Vasto Lordes amongst the Espadas.
Two were actually confirmed. Harribel and Ulquiorra. The other two had all the traits of a Vasto Lorde. Basically Starrk and Barragan.
With Ulquiorra and Harribel we were shown their stories in the anime fillers. With Starrk and Barragan we simply have to connect the dots.
When Hitsugaya was explaining the traits that defined Gillians, Adjuchas and Vasto Lordes. He specifically said that amongst the Menos Grande Vasto Lordes were the ones with humanoid size and look. To which Barragan definitely fits in. But Starrk is a possible maybe. After all we never saw how the original hollow that became Starrk and Lilynette looked like. But we do know that it was so powerful that all hollows that followed them would eventually die out given its amount of power. So tired of being alone the hollow split in two while at the same time also naturally becoming an Arrancar.
Looking at Barragan he showed all signs of being a Vasto Lorde. He had the power, the humanoid size and shape.
So at least three were definitely Vasto Lordes, while Starrk we can only guess that given his power level and skill set he was a Vasto Lorde.
Grimmjow was only an Adjucha. And never went beyond that. As was Nnoitra.
Not everything was simply straight out explicitly said. We got to sometimes connect the dots. Like for example, when Ulquiorra said to Ichigo that all Arrancars from number 4 and lower were forbidden from using their Resurrección inside Las Noches, he was in a way saying that Arrancars 1 to 4 were Vasto Lordes or at least Vasto Lorde level of power.
There was however one exception. Yammy! That guy in his natural state he wore the number 10 and in his resurección form the 10 became 0. Not just that! His size and shape was more similar to that of a Gillian. But we know Yammy was not a Gillian seeing Aaroniero (Espada 9) said he was the only Gillian amongst the Espada. So Yammy was most likely an Adjucha but like Aaroniero he too had a special power that elevated him to that rank. And seeing how easily Zaraki and Byakuya defeated Yammy then he was definitely not on a Vasto Lorde level.
At least Starrk, Barraga, Harribel and Ulquiorra took real effort to be defeated.