Why aren't V-NAND capacities 8 times greater than ordinary NAND?
The number associated with a "process node" is related to the smallest feature that can be drawn on the surface of the chip. At one time, this corresponded with the drawn width of the gate in the smallest transistors (channel length), but in modern processes, the relationship is less direct.

The spacing of the deep holes in which you construct the V-NAND structure is necessarily many times larger than the smallest feature that can be drawn on the surface of the chip &mdash; mainly in order to account for the cumulative alignment error of the large number of processing steps &mdash; so the overall area efficiency of the V-NAND technology is less than you might expect from looking at the raw numbers.