I’m so glad you qualified that. 
Not a good idea. I believe the major rental agencies prohibit taking the car off the main island, and that the ferry requires the car registration.
I have no idea if they have yet returned to Pre-Covid capacity, but preference for both passengers and cars has always been given to residents of Vieques. So even with the required reservation, you could get bumped.
Additionally, the ferry timetable is more of a suggestion. Ferries are delayed or cancelled for numerous things - weather, maintenance, the day ends in “s,” the day ends in “o.”
But Vieques is beautiful.