One and only one entity, customer. Unfortunately acceptance criteria and acceptance testing are the areas where customer does not contribute. One answer is awareness about acceptance testing and definition of acceptance criteria.
UAT test cases or checklist is prepared by organization itself based on SRS/FRS/BRS.
"Customer or vendor or client just approved these test cases/ check list to have common understanding."
Finally, when we come to UAT level then both of us means organization and customer/vendor use the same test cases/checklist. Organization use these testcases/checklist to perform internal acceptance testing by inviting the customer/ end user in org. and client/vendor use these testcases/checklist to perform external acceptance testing at his site.