Depending on the routing involved and any other relevant practical considerations, I would have thought that the best approach in that situation may well be for you to book a new ticket on the new date, using V if necessary, and cancel the existing ticket. There's no doubt that you're entitled to book for other people using V.

And depending on the nature of the date uncertainty, you may want to consider booking the original tickets with this in mind - in particular, booking two one-ways rather than a round-trip ticket - so that you don't end up in a situation in which you can't cancel the ticket that needs to be changed.