It has not been officially confirmed, but as the cero (0) Espada, he likely is. (As some assume up to Ulquiorra are Vasto Lordes. It is also highly possible that he is an Adjuchas with a very useful ability, he probably gets alot of his power from his sheer size and strength. He and Aaroniero both gain more strength as they consume more and conserve their energy, and then focus it all into their release forms.
If he is, the only person who seems powerful enough to defeat against Yammy is Captain Unohana. (of those in Hueco Mundo that is)
Either way he is still pretty strong.
Agreed. It's never been outright stated that any of the Espada are Vasto Lorde's, but whatever Yammy is, it's a freak. A hideously strong freak, at that. It has been implied that Uliqiorra is a Vasto Lorde, as the shadowed picture of a Vasto Lorde that is shown in the manga is remarkably similar to him. Or rather, his helmet. But at this point, it's all speculation.