If you need to apply a discount to a vendor invoice, That is done under the Accounts Payable Flagging menu.
Select your latest due date or discount date.
Then click get accounts payable.
The invoices that have terms for discount will automatically fill in the discount for you, if the date is within the discountable time period.
If you need to edit the discount amount , just click into the field and update the discount amount.
Then flag the invoice to pay.