If one is allowed to assume sci-fi, ... no, not even sci-fi, outright sci-fantasy levels of technology like the Matrix, the question becomes essentially meaningless. One might as well just say a new body will be instantaneously grown out of magical quantum nanobots created by the Singularity the second the brain in the vat scenario occurs and avoid any of the hard questions raised about becoming a brain in a vat in the first place.

Since none of that is likely to happen here in the real world, ever, it makes far more sense to take a hard science approach based on what is known to be at least theoretically possible today.