Thank you for your query,
Your child seems to have motion sickness
Children with motion sickness , they get vertigo and vomiting
whenever they travel in bus.
Some children will overcome this problem as the age grows.
Medication like promethazine
in prescribed dosage half an hour before starting bus usually help to decrease the vomiting associated with motion sickness.
Consult your Pediatrician
for medication advise.
Hope I have answered your query.