Vertical farming is a solution in search of a problem. It's not viable and it doesn't solve any meaningful issues in agriculture. In cities, land is more productive when used for other purposes (residential, commercial, retail, etc.), and out in the country where crops are typically grown, space simply isn't an issue to the point that you need to go up. Indeed, way more food is currently produced than is actually needed for human consumption (most cropland in the US is used to feed livestock or turned into bioethanol).