In a situation where the project has dependencies on the client's vendor management system but the vendor management team does not benefit directly from the project, it is crucial to engage and gain the support of the vendor management head. The project manager should collaborate with the project sponsor to develop a communications management plan that addresses the importance of the vendor management head's support for the success of the project. This plan should outline the criticality of the vendor management system to the project's requirements and strategize on how to communicate this effectively to the vendor management head, ensuring their understanding and cooperation.
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PMBOK Guide - Sixth Edition
The Standard for Business Analysis - PMI-PBA Guide