Build vendor delays into your risk register from day one. Have a contingency plan for the most critical vendor dependency - what do you do if they are two weeks late?
Look for: clear contracts and SLAs, regular communication cadence, risk mitigation for vendor delays, escalation paths, and contingency plans. Best candidates discuss how they protect the project from vendor failures while maintaining productive relationships and how they manage internal stakeholders' expectations around external dependencies.
Senior PM skill. Many projects fail because of unmanaged external dependencies. Candidates who treat vendors like internal teams (always available, always on time) will be caught off guard.