So let me get this straight--if you get vaccinated, you quit passing germs? Is that what you just said? I think your answer is self-isolation.
I don't have all the data, but a vaccinated person is not a COVID factory. Meaning that the virus is not going to replicate as quickly, and thus minimize, but not totally eliminate their expelling of the virus. Their spread is minimized to the point that a vaccinated person is able to fend off that diminished viral spread.
An unvacicinated person can still infect a vaccinated person with a high enough viral load to transmit COVID, and as the vaccine is only 96% effective, that leaves 4% (1 in 25) vaccinated people vulnerable.
The 70% herd immunity number comes from that's the point where the R0 is likely to drop below 1.0 so that the spread isn't eliminated, but is constantly shrinking.