No, R0=6 is a given. I'm not calculating R0. R0 for Delta is estimated between 5 and 9.5. I picked 6, just for illustration. Given a starting R0 and a vaccinati

No, R0=6 is a given. I'm not calculating R0. R0 for Delta is estimated between 5 and 9.5. I picked 6, just for illustration. Given a starting R0 and a vaccination rate and a vaccine effectiveness, that is how you can calculate effective Rt. There's nothing circular about that. There's no such thing as "R0 within a vaccinated community" - R0 is the starting reproductive number. Effective reproductive number - given various mitigation measures like vaccination, masks, natural immunity, etc - is Rt. You can try the calculations using any other starting R0, from 5 to 9.5 if you stay consistent with estimates of Delta transmissibility. The point is that it's possible that a group of vaccinated people would never have an Rt below 1 from vaccination alone, simply because of the limited vaccine effectiveness against transmission.