Updating the incident response plan to include specific actions for handling a data breach involving customer information is the best response because it addresses the exact gap identified during the audit. This targeted modification ensures that the plan reflects the necessary steps to properly manage such an incident, which could include notification of customers, cooperating with legal authorities, and conducting a thorough investigation to prevent future breaches. Adding a general cybersecurity policy is too vague, contacting third-party vendors is only a part of the response and would be included as a step within the plan, and reviewing baseline configurations is unrelated to incident planning for data breaches.