You are correct that the transmission is not equal. But the transmission among the vaccinated is also not zero. So what makes you think that even if we got to 100% vaccination (which never happens with any vaccine) that we would eliminate Covid?
It doesn't have to be 0. I mean, if it was that would be great, but it doesn't have to be.
I tried to google it, but I'm not finding any measure for the R0 rate of Delta measured in vaccinated populations.
The math is simple though.
If there are 1,000 infected people today. If statistically, each of them infects 1 person, after 7 generations of infections we'll still have 1,000 infected people. Give or take, since it's statistics and an outlier super spreader could push the bounds some, 1000 is a small data set.
If it was 3 people, that 1,000 leads to over 2 million after 7 generations. That super exponential spike we see with the steep slope up.
If it's 0.9 though, its' only 478 after 7 generations. That's clearly not 0, but it's half as many as when we started.
If it's 0.4, it's only 2 after 7 generations.
If it's 0.99, then 932 after 7 generations.
If it's 0.999, then 993 after 7 generations.
We don't need the vaccine to eliminate all transmission. It just needs to get the number of new infections per person down to less than 1. Even just barely less than one and it will drive down cases. Obviously, the lower the value the faster it happens, but any amount below one and it happens.