When faced with a situation where a vendor is unable to deliver as promised, it is essential for the project manager to engage in communication with both the vendor and the project team to explore all possible solutions. This approach aligns with the PMBOKGuide's emphasis on communication as a key aspect of project management. The project manager should work collaboratively to address the issue, which may involve negotiating a new delivery schedule, modifying the project scope, or finding alternative ways to meet project objectives.
References: Exam PMP topic 1 question 128 discussion - ExamTopics1.