8 month old vomited over 90 mins. When lie on back, she attempts to vomit again...even in sleep. What to do?

Keeps vomiting. Best is not to give your baby anything at all to drink after vomiting for at least one hour. This gives her tommy a chance to settle down. Then if she is awake offer very small amounts of pedialyte. If she is asleep, let her sleep. In 90 minutes she will not get dehydrated. If the vomiting continues over 12 hours, have her seen by her pediatrician. Hope this helps.
Could be serious. Two things have to be watched for. Is child getting dehydrated? Must check wt, mouth, skin turgor, activity level. Second thing is cause of vomiting. If just a GI flu 95% treated successfully by mouth but if a CNS infection requires hospitalization.