Welcome to Healthcaremagic.
A 20 month old vomiting
very frequently could be due to gastric upset due to undercooked food but if not subsided even after medications and a duration of 2 to 3 days then she has to be checked for physical anomalies and this can be done by an Ultrasound
I advice you to take her to a pediatrician
who examine her with similar views and plan the required management.
Wish you good health.