If you'd like to report on specific Voucher codes and balances, create a Vouchers Report. This will also show you Credit, Debit, Net and Expired Value of your Vouchers.
If you'd like to report just on sales of Voucher products, then create an Orders Report.
NOTE - As Voucher products aren't connected to performances, these won't appear in a Performances report.
Another option when looking for Voucher sales is via the Vouchers page, where you can browse and manage Vouchers.