The auditor violated the principle of confidentiality by disclosing information about the organization without approval. According to IIA guidance, internal auditors are expected to respect the confidentiality of information acquired in the course of their duties and not disclose any such information without proper authorization, unless there is a legal or professional obligation to do so.References: The Institute of Internal Auditors (IIA) - Code of Ethics and International Standards for the Professional Practice of Internal Auditing.

Demonstrating proficiency as an internal auditor is best reflected through continuous professional development. This involves adhering to The IIA's requirement for ongoing education to maintain proficiency and relevancy in the field of internal auditing. Continuous professional development ensures that auditors are up-to-date with the evolving audit practices, standards, and relevant regulatory requirements, thus enhancing their capability to perform effectively.References: The IIA's Code of Ethics and Continuing Professional Development standards.

If an internal auditor believes that the internal audit activity's independence is impaired, the first action to take should be to discuss the impairment with the audit manager. This step is crucial as the audit manager can provide guidance, support, and potentially escalate the issue appropriately within the governance framework.
It ensures that the concerns are addressed promptly and effectively within the internal audit function before reaching out to higher levels of management or the audit committee, maintaining a proper chain of communication and resolution.References: The Institute of Internal Auditors (IIA) - International Standards for the Professional Practice of Internal Auditing

This phrase from the interviewee is most likely to raise concerns regarding potential control deficiencies or fraud risks, because it indicates a lack of segregation of duties and proper backup arrangements in the purchasing process2. Segregation of duties is a key internal control that prevents or detects errors or fraud by ensuring that no single person has complete control over a transaction or activity3. Proper backup arrangements are also important to ensure that the purchasing process can continue smoothly and effectively in the absence of the department manager4. If the department manager cannot take more than a few days of vacation, it may suggest that he or she is trying to conceal some irregularities or misconduct in the purchasing process, or that there is no adequate supervision or review of his or her work5.
References:
1: Internal Audit Interview Questions & Answers - Wisdom Jobs
2: Segregation of Duties: A Key Internal Control - The CPA Journal
3: Segregation of Duties - The Institute of Internal Auditors or The IIA
4: Backup Arrangements - The Institute of Internal Auditors or The IIA
5: Fraud Prevention Checklist - The Institute of Internal Auditors or The IIA

The control that would have likely prevented the fraudulent modification of vendors' banking information is management's approval being required for updates to vendors' banking information. This control would provide a layer of verification and oversight, significantly reducing the risk of unauthorized and fraudulent changes.References: Best practices in vendor management and internal controls over payment processes, as advocated by The IIA.