You mean Rt, no R0, since R0 represents an inherent property of the virus while Rt represents current local conditions and spread.
But you are talking about herd immunity, which we know does not apply to this virus, vaccine or no vaccine, since infection doesn’t provide sterilizing immunity and people have substantial risk of reinfection within a year
Our group (PolicyLab) modeled and published Rt in every county in the nation throughout 2020 and I assure you it was absolutely not the case that Rt<1 when the vaccines rolled out nor was it true that vaccine rollouts increased Rt.
The Rt was very high in many places in late fall and early winter as many places that didn’t experience major waves in spring or summer 2020 experienced their first large waves (waves have locally high Rt) in fall to winter.
Sorry but on this one you are REALLY far off (and if you think I am wrong please show me the data/studies substantiating your claim that Rt was less than 1 all around and increased locally at a place when they had their vaccine rollouts, and better yet some evidence that vaccine rollouts increased Rt)