A flat farm, everything else being equal, will take less humans. There's nothing about being indoors and vertical that is a magical advantage for mechanization. There's also a lot more mechanization necessary in dealing with vertical farms. All of a sudden you have to transport soil, fertilizer, and harverst vertically in addition to horizontally.

Already, wheat fields can be tilled and harvested by autonomous combines. Direct human labor is basically down to picking fruits and vegetables. Most of the time, it's going to be cheaper growing something outside, in the sun and dirt that's already there.