After contracting a vendor to build a system, the acceptance tests show unsatisfactory quality. How should the project manager substantiate the claim to the vendor?

The most effective way to substantiate a claim of unsatisfactory quality is to provide objective evidence. Presenting the failed test results directly demonstrates that the system does not meet the specified requirements. Comparing these results against the agreed-upon acceptance criteria, which were established during planning and are part of the contract, clearly shows the vendor's deliverable is deficient. Sharing unsuccessful results from previous projects is irrelevant to the current vendor's performance and does not provide evidence for this specific contract. Documenting issues in the risk log after the fact does not prove the vendor's failure to meet acceptance criteria; it only notes a risk. Showing that acceptance criteria were changed by the vendor without approval would be a valid claim if true, but the question implies the issue is with the quality against existing criteria, not a change in criteria.