Ok. Most transmission takes place in the home. So compare in home transmission rates between vaccinated, unvaccinated, and partially vaccinated households.
In case you‘re wondering, someone already did the study. Turns out, the vaccine reduces transmission rates, just like the biology would predict:
Vaccination reduces the risk of delta variant infection and accelerates viral clearance. Nonetheless, fully vaccinated individuals with breakthrough infections have peak viral load similar to unvaccinated cases and can efficiently transmit infection in household settings, including to fully...
www.thelancet.com
Clearly it is not impossible to calculate. They did calculate it, and it passed peer review.
Which leaves you in the position of arguing “Yes, but that study isn’t actually valid. I am a person on a soccer forum, and that’s just a peer reviewed study in Lancet with multiple citations. They don’t know as much as I do.”