Mattn, there currently is not a ferry from Tortola to Cruz Bay. If the West End opens as announced in August you may be able to do this if Cruz Bay also opens a ferry custom area on St John.
You will need to go one of the follow routes:
Road Town to Red Hook to Cruz Bay
West End Tortola to Jost to Red Hook to Cruz Bay
We are land based on Jost and took the 9 am Inter Island several weeks ago to Cruz Bay. Easy!