Hi, I have read your query, & understand your concern. Post chicken pox vomiting is very rare , as you have rightly said I see 2 possible reasons- - hyper acidity due to the drugs given to control fever, - baby not eating well due to lac of appetite following the infection( B complex deficiency) leading to raised acidity. please take the baby to children specialist for examination & advice of medicines, In case baby is dehydrated, oral replacement therapy or IV drips are needed. Usually metoclopramide syrup is given in the beginning and mild,bland liquid diet is started to feed baby in beginning.followed by gradual introduction of normal food. Lactobacillus preparation is added for normalising digestive process. Thanks.