Main issue will be what ferries are running on the specific day you will be travelling. The ferries out of Road Town take 1 hour to get to the ferry dock in STT. Then add one to two hours to get through both USVI and US customs/immigration (yes, you go through both in way back from BVI) and security, even with GE. Right now there is no Sopers Hole to STT ferry right now. May change, but I would not count on it.

If you take a water taxi, takes a total of 2.5 -3 hours from being picked up to the airport (includes going to St. John-Cruz Bay for USVI C&I, then being dropped at Red Hook Then taking taxi to airport). Then you go through US C&I at airport. Saturdays are busy even in low season.

Best advice I can give you to avoid stress - just go ahead and take early transport from BVI to USVI, eat lunch at or near airport and take it easy. It is a travel day. Cutting it close, getting sweaty and bothered not best way to end vacation, imo.