As far as I know vinegar is added to slip ( or slurry) to thicken the
clay so as to make it more compatable with the item being repaired. It
will slow the drying time of the repair(cover the repair during drying).
This is to be used only with greenware-preferably damp. As far as
adding sodium silicate- it would acomplish the same thing. It is
changing the charge of the electrons. Adding sodium silicate causes the
electrons to repel each other allowing the slip to be more fluid. I
believe too much has the opposite effect-and binds them together as in
the case of the vinegar.
Read the ceramic students bible by Rhodes for more accurate details.