When we were on Vision we booked for next year March (back to Freedom!) and the biggest thing I remember was that it was $100 per person deposit instead of their usual bigger deposit. There was also an OBC for the next cruise.
However, that last bit has changed. Or CAN change, if you wish. I believe the new option is to make the future cruise deposit one that cannot be refunded, but you get that amount in OBC put onto the *current* cruise. Which I think would mean you can use the OBC as your deposit for the future cruise? Gets a little circular in my head, but that's my understanding of it. But since you've already gotten that OBC to use; it's gone now, it makes sense that you can't get it refunded if you cancel the future cruise. Make sense?
Get a sense of what you might want, that's my advice, and sign up early. We nearly missed getting to book onboard, because we kept putting it off.
Oh and I am 99.99% sure you can book onboard RCI for Celebrity, which is nice.