We've brought frozen meat in for Kevin Campbell, our taxi driver, and no Customs issues since St. Thomas is a U.S. territory.
You don't need to check out of the USVI, and depending on the # of days you'll be in the BVI, you can check in and out at the same time. You'll need to stop at Cruz Bay on St. John when coming back into the USVI. So, I would hit the snorkeling spots on the way to the BVI.
Diamond Cay on JVD is a turtle breeding ground and we normally see tons there.