Hi Mark

Vesta can't be a mere core fragment as it is covered in lava i.e. basalt. Thus we're seeing close to original surface, within the first few million years after solar system formation, before all the Al26 heating has finished. The HST images clearly show a massive crater on one of its poles, and there's a whole family of asteroid fragments with spectroscopic affinities with Vesta, so Vesta is clearly a parent body and not a remnant.

As for icy vs rocky I don't think one can exist without the other in the outer solar system without some sort of major heating event to dispose of the ices, or some binary fission event to separate the ice from the rock. And even then it's not too efficient - Charon has a density of 1.73, which means about 50:50 ice:rock, while Pluto is about 25:75 ice:rock, so their fissioning event wasn't especially efficient at casting away the volatiles.

Maybe. So many Saturnian moons seem to be nearly all ice, while Callisto and Ganymede are almost 50:50 against my naive expectations. Planetologists have a lot of mysteries to ponder yet ;-)

Adam