Vinegar does work (about 80% of the time). As does Acetic acid. What an
acid does is reduce the amount of shrinkage that occurs as the clay dries.
This is often enough to prevent a cracking when repairing a chip off a
greenware piece. At school, because I never seem to have Vinegar, we use
photographic stop bath. Handles can be replaced, chips placed back, and
cracks filled with a clay slip made from moist clay and an acid. I don't know
why it doesn't always work, but we've had overall success with the process for
the 25 years I've been teaching pottery.