A required input for a deliverable was missed because of a miscommunication with the vendor, extending the project's critical path. What should the project manager do next?

The project manager should execute a contingency plan because a critical path issue has already occurred. Contingency plans are pre-defined actions to be taken when identified risks materialize. Since the missed input has extended the critical path, it's an immediate problem requiring a pre-planned solution to mitigate further impact. Initiating an audit or reviewing lessons learned are valuable activities for future prevention and process improvement, but they do not address the current, active critical path issue. Performing a detailed analysis of vendor-related issues is a reactive step that might be part of a larger problem-solving effort, but executing the contingency plan is the most immediate and effective action to resolve the current critical path delay.