I think you could probably make this happen just by creating a view that:
Groups records by “2022 Sales Cohort Month”
This will give you a total number of vendors in that cohort as a number of records in that group
Uses the “% Filled” summary formula on the “Date Completed” field
This will produce the % of Completion you are looking for in the grouping bar for each Cohort Month group by showing the % of records in that Cohort Month Group that have a date filled in for the “Date Completed” field
Thanks, Jeremy. I should have mentioned that one of the end goals is that the % can be a field tied to the vendor, so that we can export the csv with that %. Knowing that, do you have a different suggestion?