A complimentary solution with vertical farming is sub-acre farming. A sub-acre farming method now being practiced throughout the U.S. and Canada is called SPIN-Farming. SPIN stands for S-mall P-lot IN-tensive, and it makes it possible to earn significant income from growing vegetables on land bases under an acre in size. SPIN farmers utilize relay cropping to increase yield and achieve good economic returns by growing only the most profitable food crops tailored to local markets. SPIN's growing techniques are not, in themselves, breakthrough. What is novel is the way a SPIN farm business is run. SPIN provides everything you'd expect from a good franchise: a business plan, marketing advice, and a detailed day-to-day workflow. In standardizing the system and creating a reproducible process it really isn't any different from McDonalds. So by offering a non-technical, easy-to-understand and inexpensive-to-implement farming system, it allows many more people to farm, wherever they live, as long as there are nearby markets to support them, and it removes the two big barriers to entry - sizeable acreage and significant start-up capital. 
While vertical farming will still take some time to get off the ground, sub-are farming is already showing how agriculture can be integrated into the built environment in an economically viable manner. You can see SPIN farmers in action at www.spinfarming.com