When you check in at the kiosk (don't check in online), you will get the option to print a receipt. When I did this, on my ticket which was fully paid by the $325 voucher, the fare calc line read:
PHX US X/PHL US PVD 120.00KXA0NJ6P/T325US X/WAS US PHX 120.00KXA0NJ6P/T325.00 USD0.00END

I'd imagine your scenario would look similar, but the USD0.00END would be replaced by whatever was the fare paid over the voucher amount.