I don't know anything about airfare prices between islands. But, just because you want to charter
in BVI doesn't mean you have to do your other week at a resort there. There are many resorts/vacation rentals in places like St. Maartin, St. Lucia, etc. which might be more attractive. If you stayed in St. Thomas in the USVI, you could take a ferry
If instead you did a 2 week cat charter
in BVI, I would think that in addition to BVIs they could take you to St. John or St. Thomas in the USVI as well - so either way you get to see more places.