That lump is probably an accreted mass that impacted with enough force to stick to the rest of Vesta but not to shatter the accreted mass into bits. The weak gravity would not have been enough to cause the complete absorption of the mass, so there it sits today. On display sort of as a warning to other asteroids. (Warning of what? I don’t know!)
It certainly gives that impression, almost as if it was a blob of clay, but I'm not sure how that fits in with the physical models that they have for this sort of process. I wonder how much of puzzle this image presents. They always brag about how they love surprises, but maybe they didn't really expect a surprise here, or maybe they'll just say this is what they expected anyway.
According to this article, it accreted and "evolved" before being bombarded. I don't see anything accounting for an accreted lump. They show a model, pre-encounter of course, that shows a bump at the bottom. I think this is supposed to be the wall of the crater at the south pole that they talk about. I have trouble lining all these things up. For instance, it's oblate, so isn't the south pole on one of the flat sides? The new picture must have the poles to the right and left ... right ? You have to dig for this stuff.