I don't know about other airlines, but the WN website has this to say about vouchers and gift cards:
Southwest LUV Vouchers are valid as a form of payment toward future air fare only on Southwest Airlines through southwest.com, toll-free Reservations at 1 800 I FLY SWA (1 800 435-9792), or at any Southwest Airlines Ticket Counter.
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http://www.southwest.com/luv_voucher...onditions.html (emphasis added)
A southwestgiftcard can be used to purchase Passenger travel on Southwest Airlines. Sales tax, security fees, Passenger Facilities Charges, Pet Fare, Drink Coupon Books, Unaccompanied Minor service charge and oversized or excess baggage may also be paid for with a southwestgiftcard. The gift card is not valid for the purchase of Southwest Airlines Group Tickets, Southwest Airlines Vacations’ Packages, Rental Car, Hotel, Cruise, Cargo, Southwest Airlines merchandise, codeshare bookings (on codeshare partners’ web sites) or inflight purchases.
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http://www.southwest.com/gift_cards/gift_card_faq.html (emphasis added)