As the Dawn probe has gathered information on Vesta, several things have become apparent. Something happened that rendered the bottom of Vesta totally flat, even though it's large enough for gravity to press it into a sphere. The crater is unlike anything seen before in the solar system and scientists have yet to determine a cause. There are also questions about whether Vesta demonstrates evidence of past volcanism. Luckily more information will be forthcoming as Dawn will get closer in the coming months.
http://www.irishweatheronline.com/news/space/asteroids/the-canyons-valleys-and-mountains-of-asteroid-vesta/38220.html
http://blogs.discovermagazine.com/badastronomy/2011/09/17/vestas-odd-bottom/
