I’m on my phone, but if I did this right.Well, R0 for any variant has a great deal of variance. But the best estimates I've seen put the original strain at 2-3 and the Delta at 5-6.
The effective reproductive number, Re, is defined by Re = R0(1-x*v), where X is the % of population vaccinated and V is the vaccine effectiveness.
If we take R0=6 and look only at the vaccinated population (x=1), then if the vaccines are 90% efficient we get Re = 6(1-.95) = 0.3, a very good number. If vaccine efficacy has waned to 70%, we get Re = 6(1-.7) = 1.8, not so good.
Now, if we take only 75% of the population vaccinated at 70% we get Re = 6(1-(.75)(.7)) = 2.85. Pretty horrible. If boosters would get that back to 95% then we'd be up to Re = 6(1-(.75)(.95)) = 1.725, still not enough to control spread.
If we get 91% vaccinated and the vaccine holds at 92% effective. Say after a booster hopefully it holds, maybe a fourth. Then, Re goes under 1 and Bob’s your uncle we hit the end.
Alternatively, we have to keep doing other mitigation to help the vaccine get us just under 1. What a pain.
Even small changes have an impact.