Sure, as long as they have a card reader. A few of the smaller mobile carts do not have readers, nor do the roving vendors that can be found in the seatig bows before Fantasmic and some of the other shows.
Disney Gift Cards can be used at any Disney-owned venue that has a card reader in the parks, resorts, water parks, and Downtown Disney - basically, anyplace that you can use your room key to charge stuff.