1. UCLA research suggests ending mask mandates for vaccinated people is perhaps the strongest incentive for vaccine-reluctant people to get vaccinated. Nobody (or, no normal people) like masks. Providing masklessness as a reward for vaccination is good policy.
"The incentive to stop wearing a mask and social-distancing in public also had a strong [effect on willingness to get vaccinated]. On average, relaxing the mask and social distancing guidelines increased vaccine uptake likelihood by 13 points. The largest gains came from Republicans, who reported an 18-point increase in willingness to get vaccinated."
https://www.nytimes.com/2021/05/04/upshot/vaccine-incentive-experiment.html
2. Ending mask mandates in businesses provides an incentive for vaccine-reluctant shoppers and workers to get vaccinated. Remove the notion that you are "protected" by the masks around you, and the incentive to get yourself protected goes way up.
tl;dr: Maintaining mask mandates for vaccinated people is providing the wrong incentives and is hindering the vaccination effort.