You should understand the point that there are 2 doses of varicella vaccine out of which your child have received only one. That means half immunity. Also it is difficult to say at first look that this is chickenpox, due to evolution of disease with time.
Hence I would advice to confirm the same with a nearby pediatrician or skin specialist. ( You can share a few pics here also for us to confirm)
Even if it is chickenpox, due to one dose of vaccine the kid has received - lesions and illness should be a mild one.
Next Steps
Confirm the disease as chickenpox, can start applying lactocalamine lotion 2 times a day over lesions, for soothing effect.
Health Tips
Must carry your vaccination card while visiting a doctor