I'm a little confused over what you are trying to do here. The Voucher is not a payment, it is a representation of the Vendor's invoice.
The typical process for paying Vendors is, when a vendor's invoice is received to enter that as a Voucher. Then it is selected for Payment (I use the Accounts Payable Workbench to do all of this), and then a Payment is created, The Payment is where things like check numbers are entered or generated, Similarly the payment also represents electronic (wire) payments. If you use the payment you will not run out of check numbers.