By far the easiest thing to do is stop in Culebra (or Puerto Rico proper; bigger variety and much cheaper provisioning) and clear into the US there. Culebra has a small customs/immigration office within walking distance of the dinghy dock. And not a bad place to visit anyway, before heading off for the states.
The USVI is a duty free port so you are not in the US. But Puerto Rico is a part of the US and by clearing in there that makes it a "coastwise" voyage to the states, requiring you only to notify customs of your arrival in the continental US by phone. You'll have to pay a (I believe) $71.00 fee, but you'd have to pay that in the US upon arrival from a foreign port, anyway.
If you've ever flown to the US via Puerto Rico you know what I mean. You clear in there and there's nothing to do upon arrival in the US.