You can use the Vieques "Publicos" for a reasonable fee (ask the driver). 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.
You can make arrangements with any of the drivers to pick you up at specified times for an additional cost. The drivers are friendly and willing to get your business. Here are a couple of them:
Vieques Taxi: 787 741-TAXI(8294).