*Another ahem*
We agree that Shinigamification gives the Hollow a power boost, correct? (Addition abilities such as the Hierro, as well as the resurreccion and the sword and stuff).
Now, most people would agree that Espada 4-1 (both inclusive) are Vasto Lordes; based on their very human like released forms, as well as based on the fact that Ulquiorra mention "Espadas 4 and above cannot release inside the dome of Las Noches".
It seems likely from this (and probably other info) we can agree that Ulquiorra, Hallibel, Barragan and Starrk are Vasto Lorde.
Now of course, it is unlikely that all Hollows, when undergoing Shinigamification, will gain the same boost in power (let's say we have 2 Hollows, ranked 2 and 10. They probably won't be boosted to 5 and 13, respectively. You'd think the 10 would be boosted further).
So, going back to your original question, a Vasto Lorde, when Shinigamified, will likely become somewhere around the rank of 5 or above, due to other (likely) Vasto Lordes being Espadas 4 and above.
*Does so*