Tier Harribel, a Vasto Lorde managed to go toe-to-toe with Captain Hitsugaya. There is no mention of how a Hollow becomes a Vasto Lorde. Grimmjow Jaegerjaquez speculated that an Adjucha could ...

Ulquiorra was once a Vasto Lorde-class Hollow ... with his hidden power, the true number one? The Gotei 13 Captains are incredibly powerful but how do they fare against each other?