You can use the Vieques "Publicos" for a reasonable fee. You can recognize them by the "Vieques y sus barrios" sign across the windshield. Most of them are large vans that accommodate up to 10 passengers. They are easy to locate at the ferry harbor and the airport.
The "publico" fares are regulated:
Vieques Taxi: 787 741-TAXI(8294).