I'd suggest the Road Town Fast Ferry which departs Charlotte Amalie at 4:15 and arrives Tortola at 5:05. Then take Speedys 6:00 pm which gets you to VG at 6:30. Good news is you'll have time for a late lunch at Gladys' in Charlotte Amalie (Royal Dane Mall).
I would take the first ferry you can out of CA to Roadtown- if you get there early and clear customs quickly (which does not always happen) you can have lunch in RT there are plenty of places across from the ferry dock, or you can have a bite to eat upstairs at the ferry dock in CA. They all leave out of the same place and it is very close to the airport. If you get your luggage quickly you can take an earlier one.
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