"Roaming" is an old term that used to mean that you were roaming on a roaming partner's tower (and probably paying $5/day for the "privilege"). Now it just means that the tower is identifying as something other than Verizon. As long as you have something in writing from Verizon that says that there's no charge for using the phone in the USVI, there is none.
Sprint is a USVI CDMA carrier, so you may be "roaming" on their towers (it depends on with whom Verizon has a "roaming" contract).