You also have to take into account population density and mask adherence within those clusters, as well as propensity for private gatherings which might be unmasked and dense.
In NJ, Mercer County generally has better mask compliance than Monmouth County and their numbers reflect that. They are about the same vaccination-wise, but they also have dramatically different populations.
My congregation has been masked exclusively since reopening in July 2020 and we have had zero COVID spread. We are also highly vaccinated. We have also had lower on-site attendance in a very large, drafty, well-ventilated building. Another congregation had less mask compliance and more COVID, but a smaller building where people sat closer together and wasn't as well ventilated. Basically, for us, with Omicron chomping on NJ like Pacman eating pips, if we get to 10 days past Christmas with zero COVID spread (high attendance, high interaction including a pageant), I will assume masks saved us. Then again, a lot of people feeling symptoms attended online instead of onsite, so human wisdom may have also saved us.
It would be a very interesting comparison, but I think it would have to be a carefully designed study with lots of nuance.