There are two ways you can pay to a vendor credit.
1. From the vendor's page. (WP Admin Dashboard → WP ERP → Accounting → Vendors)
Hover on the vendor, click on Payment Voucher.
2. From the expenses page. (WP Admin Dashboard → WP ERP → Accounting → Expenses)
Click on the button Payment Voucher.
For both cases, this voucher form will be created. Select the vendor and the due amount will be automatically loaded. You may also select if there are multiple credits.
Now if you go to the vendor's page, you will see that the paid amount has been debited.