There's two types of vouchers. The one that's meant to be used at the airport vendors says:

** GRATUITY NOT INCLUDED **
REASON FOR ISSUANCE - <reason> BILL TO DELTA AIR LINES INC.

The ones that have a card type and card number listed (like credit card info) can be usable at a hotel restaurant, should they choose to take it (they'd just have to run it like a manually entered credit card).