The best criteria for monitoring an IT vendor's service levels are the performance metrics, as they provide quantifiable and measurable indicators of how well the vendor is delivering the agreed-upon services, such as availability, reliability, quality, timeliness, and customer satisfaction. A service auditor's report is a document that provides an independent opinion on the vendor's controls and processes, but it may not reflect the actual service levels or performance. A surprise visit to the vendor may help to verify the vendor's compliance and operations, but it may not be feasible or effective for monitoring the service levels on a regular basis. An interview with the vendor may help to obtain feedback and insights from the vendor's perspective, but it may not be objective or reliable for monitoring the service levels. References: CISA Review Manual (Digital Version), Chapter 2: Governance and Management of IT, Section 2.4: IT Service Delivery and Support