The USVI is a US posesion so you don't need a passport or go through C&I on arival. Arriving in the BVI you do need your passport and will clear at West End or Road Town. Kevin is the best way to in sure a smooth transition from airport to ferry.

On the return you clear into the USVI at the ferry dock at Charlotte Amalie or Cruz Bay if going to Red Hook. At the airport you will clear into the US proper but I understand they have eliminated the forms recently. (Can someone confirm?)

STT can be a real PITA on weekends so hiring a skycap is a good idea if you have checked baggage and time is tight. To them, time is money so they have ways of shortcutting the lines.