One of the risks you are waiting for the project was late delivery of prototype from vendor. You adjusted project schedule to minimize impact of risk, built in penalty for late delivery, and developed action plans in case vendor delivered late. You also recognized risk with vendor, that they have very little technical depth. If main engineer is not available to the project, risk of a delay is even greater.
describe how you will interact with vendor in managing these two risks:
How will you monitor engineer's availability risk?
How will you determine vendor's progress?
What information will you ask them to share with you? How often?
How will you know if circumstances is becoming critical? describe what will you do?