Yes, when you make a V&A reservation it can be cancelled without penalty. If you are able to get CT just tell the reservation person you had a dining room reservation and it will be cancelled.
Also, if you were not aware of it, you can get the same special tasting menu (and wine pairings) as you would at the CT in ghe dining room. However, it must be ordered in advance, the price is the same as for CT, and you must be at the early seating.