Re: Bvi - usvi- svi
If you really miss the BVI, but want to sample the USVI, you can split your charter between the USVI and the BVI. Assuming you are starting from the USVI, you don't have to check out of the US, but you'll need to check in to the BVI, where you have to check out, but you do this at the same time. It isn't a big deal, but you'll pay for BVI cruising and park permits. You can check in at Sopers Hole (ferry terminal) or Jost van Dyke. It shouldn't take very long. We've checked in at Spanish Town, and that was more time-consuming, perhaps because they don't see as many charters.
Coming back to the US, you'll check in at Cruz Bay. The easiest way to handle this is to take a park service mooring between Lind Point and Caneel Bay and dinghy in to Cruz Bay--and don't forget your dinghy lock. The US insists you check in before landing on US territory and that everyone shows up (unlike the BVI), but it's free, unless you've tied up at their dock (which you can do, but isn't worth it, IMHO.)
Check with the charter base about the procedures for the SVI. As FarCry has said, its several hours from cruise ship heaven (Charlotte Amalie). If your charter boat doesn't have a chartplotter, you ought to bring a portable one of your own. I have used a Garmin Oregon with the Eastern Caribbean chart chip and it was well worth it for the peace of mind. The reefs around Culebra and Culebrita demand your attention.
The Seaworthy guide to the Virgin Island is recommended if you go the the SVI, but note that there are areas at the southeastern end of Vieques that may still be off-limits, despite what your guide or the anchor symbol on your chartplotter indicate. In particular, Bahia Salina del Sur had a buoy in 2012 indicating it was a "no anchor" area due to an unexploded ordnance hazard.
The good news about the SVI is that it is relatively uncrowded and very laid back. There are numerous public moorings and they are free. Fuel and water are inconvenient, unless you enjoy ferrying jerry cans, which the charter bases don't provide. However, you can buy jerry cans at the chandlery in Dewey on Culebra, where you'll also find decent provisioning and a number of restaurants. Your next best bet for restaurants is Esperanza on Vieques, which has a room to anchor but very poor holding. We gave up and picked up a mooring in Sun Bay, but be warned: it's a long dinghy ride in the dark when you leave the restaurant after dinner.
The SVI are blessed by the relative lack of development, which means there is a lot of natural scenery and few other boats, compared to the BVI. So if your baseline is the BVI, you might start off by sampling St. John first. If you like the relative quiet of St. John, you might then try the SVI on your next charter. BTW, the return trip from Culebra or picture-perfect Culebrita is an uphill slog. Forget your sails and use the motor, unless you have plenty of time and like to beat to weather for hours on end.