Car rentals should be made at the time of your lodging. If you are going to Vieques between December and April you should make them as early as possible. The reason for that is there are many more rooms and vacation rentals in Vieques than there are car rentals. If you're living in the hills away from town, you'll need to eat. Without a care you are not going to be a happy camper. The hills are steep and the roads are narrow.