I would always give triple dose of cortef right after the vomiting episode.  If this stays down - we are good to go, if she continues to vomite - I would give an injection.  I have only had to deal with more that this once in my daughters life - she was 2 and vomiting continued and I could tell she was dehydrated - and she would nt take liquids. We went to ER where she was given IV fluids.   I have been very lucky, my daughter doesnt get sick often.