You need a lot of area for that - incident sunlight is proportional to land area. To get the same amount of sunlight, you'd need to cover around the same square footage of land as there is area in the vertical farm with mirrors, and then cleverly redirect that back into the vertical farm - kind of like a solar power tower.

What would be even cleverer is to just plant the crops where the sun falls in the first place, and just do one layer of crops directly in the soil already present in that location. This would obviate the need for the multi-story tower, mirrors, soil transportation, etc. A "non-vertical farm," if you will. I think it could be a real money spinner!