I did manage to get the plaster out. I micro waved the bowls for under a
minute to warm the Vaseline and worked them to break the suction and also
wacked them on the bottom with a hammer. That was the problem for sure...but
the rigid plastic bowls were a no go. One got dropped and the bowl cracked
and a big piece broke off, the plaster was fine and popped out easily after
that! So I just took the hammer to the rim and cracked them all. So much for
using those bowls again!
thanks everybody for your advice! Next time I will grease them up AND put a
layer of cheesecloth down to make it easier to pull out the plaster.