That was my question - how much less do those who are vaccinated and contract the virus infect other people than those who are unvaccinated and contract the virus. That can't be calculated from a single R0 number.
Vaccinated group:
Rt = 6 * ( 1 - (1 * .8)) = 1.2
Unvaccinated group:
Rt = 6 * ( 1 - (0 * .8)) = 6
The question of "how much less" is hard to parse, but the point is that even among the vaccinated group (in this scenario), the virus is still spreading and growing exponentially.
I suppose if Rt was clearly below 1 for the vaccinated group, that might mean something, but if the groups intermingle, that advantage would be lost pretty quickly.
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.
Rt = R0 * (1 - (vacRate * effRate))
So if Delta R0 is 6, a community is 65% vaccinated, and a vaccine is 80% effective against transmission, then effective Rt would be:
Rt = 6 * ( 1 - (.65 * .8)) = 2.88
Real effective Rt includes impact of mitigation levels and natural immunity. So for instance, in Portland OR, Rt is about 1.4. We didn't get hit as hard as surrounding states in the first few rounds, so we don't have as much natural immunity. In contrast, Seattle got hit harder early on; their effective Rt is a little lower - and Silicon Valley is actually pretty close to 1 right now.