The most appropriate conclusion for the auditor to include in the audit report is that the organization had weaknesses in its review process which allowed questionable transactions with some vendors. This conclusion directly addresses the identified issue of questionable vendor documentation and implies that there are control deficiencies in the review process that need to be addressed to prevent such occurrences in the future.
Reference:
IIA Standards: 2410 - Criteria for Communicating
IIA Practice Guide: Reporting and Monitoring