To ANSWER THE QUESTION there is 30 % chances of spread of chicken pox
even to a vaccinated person, but the main advantage to take vaccine
is it will protect about 95% vaccinated person from serious complication or disease.
Clinical trials have demonstrated that varicella
vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella. And yes it is true that it passes through the fluid from the blister
but however a person is careful about not to spread, the distance from grandkids in essential to reduce the chances to have even mild to moderate infection with chicken pox
Please keep grandchildren away from you as much as possible till the blisters are healed completely.
hope it helps.