MCPayne, the USVI (St. Thomas, St. John, St. Croix) are exempt from the Passenger Services Act and are treated as "nearby foreign ports" for purposes of the Act. Just one of many intricacies in this legislation...makes my head spin!
I, too, would feel awkward about not having envelopes to give to the crew. I can see how vouchers can be a pain, though. If RCI wants to give pax a choice (which I applaud, actually), perhaps there can be a box on the envelope to check if pax had pre-paid the gratuities. It's been a long time since I've cruised on RCi...maybe they do this already. I also like Dave's idea of the different colored envelopes. (Actually, that's a better idea as it would be more difficult to stiff the crew.)