Never heard of anywhere round here that was specifically vegetarian, of course the simple answer if they are non meal eaters is to eat on the coast, any decent restaurant in places like San Carles will serve mostly fish or seafood and so meat is nowhere to seen.  In San Carles try any in and around the port area, la Varadero is good if a little pricey.  The tourist menu is a good sample of local seafood fare.