According to the PBA Guide, retrospectives are a technique to review the project performance and identify lessons learned, best practices, and improvement opportunities1. Retrospectives are especially useful in agile projects, where they are conducted at the end of each iteration or sprint2. By implementing retrospectives with the vendor, the project manager can ensure that the project team and the vendor share the same understanding of the project goals, requirements, and deliverables. The project manager can also facilitate knowledge transfer and collaboration between the team and the vendor, and address any issues or risks that may affect the project continuity. References: 1: PBA Guide, page 154 2: PMBOK Guide, page 188