In the anime/manga series Bleach, Vasto Lorde is said to be the strongest form a Hollow can take, and are stronger than even a captain of the Gotei 13 (I'd assume they're comparing your average captain to your average Vasto Lorde, but we can never be sure). Now, we know when Hollows become Arrancar they get stronger, so it's no surprise that when fighting the three strongest Arrancar in the series (all of whom are implied or outright stated to have been Vasto Lorde before their assent to Arrancar), that multiple captains, Lieutenants and similarly powered Vizards have to fight them in order to win.
However, that being said. All of these Hollows were Arrancar when they fought in the battle of Karakura and had been for quite some time. If this is the case, then that would suggest if they were merely Vasto Lorde, the Soul Society wouldn't have had nearly as much trouble. I have no doubt the Vasto Lorde would have been able to go toe to toe with a captain, but there is a big difference between going toe to toe and winning.
So with all this in mind, do you think the stories of how powerful the Vasto Lorde are were exaggerated? Or could it be possible that as far as Vasto Lorde go, the three (possibly four) under Aizen's command weren't as strong as Vasto Lorde of the past?