Japan has about half the arable land per person as the Netherlands [1] but the Netherlands is the worlds #2 food exporter [2]. Japan could probably be more or less self-sustainable in terms of food production, if they made it a political priority, even without any fancy vertical farming. In fact, they already import less foodstuffs than German, at about 1.5 times the population [3] (to be fair, Germanys central position in the EU's free trade area makes importing food a cheap thing to do).

[1] https://data.worldbank.org/indicator/AG.LND.ARBL.HA.PC?locat... [2] https://www.investopedia.com/financial-edge/0712/top-agricul... [3] https://oec.world/en/profile/hs/foodstuffs