The effect needs to be strong, because in ordinary times, many social circles cross paths and a much higher level of human contact occurs. The average person won't read into the nuance of 'the vaccine is X% effective, depending on risk factors Y and Z'. They'll rush to their ordinary behavior. While there isn't a vaccine, it's easier to obligate folks into social distancing. That strategy won't exist anymore once we have a vaccine, regardless of its efficacy.
I agree, but that's what the challenge is. We have to solve for the values of X, Y, and Z where we're comfortable with people's inevitable rush back to their ordinary behavior, and it's extraordinarily hard to figure out what those values should be.
