"... rain and from evaporation of standing water. Of course you'll need an atmosphere to encourage the recycling process... rain."
CO2 makes an atmosphere, and is also with comets.
Besides, hot water becomes a gas.
"Over time, rain helps to brake down rocks, disolves them. In the process, sodium is produced and when evaporation occurs, you have salt. It is also found undergound in caves in the form of rocks after water from rivers percolate down and long enough for the rock salt to form."
Salt requires water, steady amounts of water over time.
Why didn't something like that happen on the Moon? Impact events may have briefly created liquid water on the moon, but I highly doubt that the moon had actual periods of rain long enough to produce salts and I highly doubt that water could be on the lunar surface long enough for salts to occur in ponds. The moon would have needed a dynamo to support an atmosphere."
So, say there is less salt, and less water and the water lasted for shorter times, but you would still have a similar process.
"I don't think anyone knows the full stories of Ceres and Vesta. Although Ceres is spherical, it is very small. Was it captured late by the asteroid belt after it formed? Same for Vesta?"
That they could come from further out and migrated inwards is possible, but Ceres has apparently a lot of water in terms of it's abundance relative to other material- 25% of upper crust, so superheating it's surface with any impactor should create a fair amount water vapor.
Ceres at the moment is thought to have a very thin atmosphere of water vapor:
"Ceres may be surrounded by a tenuous atmosphere containing water vapour."
http://en.wikipedia.org/wiki/Ceres_(dwarf_planet)