My 2 nephews who were vaccinated did get chickenpox, it was a reasonably mild dose.  My 6 year old SWCAH daughter got them at 18 months, she wasn’t vaccinated, but no side effects, just the spots and a little bit of itching.  My other 2 children, 5 and 3, have not been vaccinated and haven’t had them even though they have been in contact with their cousins and other kids at kindy and school who have gotten them.