Final answer:
The auditor using the vouching audit procedure is primarily concerned with the occurrence of transactions when testing classes of transactions.
Step-by-step explanation:
When the auditor uses the audit procedure vouching, she is primarily concerned with the audit objective of occurrence when testing classes of transactions.
Vouching involves examining documentary evidence to support the occurrence of recorded transactions. It aims to ensure that the transactions actually took place and were properly authorized and recorded.
For example, in the case of cash receipts, the auditor may vouch each receipt to the corresponding source document such as bank statements or customer invoices to verify that the recorded transactions actually occurred.