Let’s take the total cropland in the U.S., 335 million acres, and subtract all the acreage (in millions of acres) that is not necessary for a vegan diet:
34 — corn for ethanol (biofuel)
34 — corn for livestock feed in the U.S.
15 — corn for export (which is mainly used for livestock feed overseas)
36 — soy for export (also for livestock feed)
36 — soy for livestock feed in the U.S.
11 — cotton (lint and cottonseed oil)
34.5 — sugar beets for refined sugar
0.9 — sugarcane for refined sugar
200.9 million acres of land
Then we would need to add back 45 million acres for vegetable oil to replace the lost soybean, corn, and cottonseed oils. The total reduction in cropland use is 155.9 million acres, which is 0.535 acres per person in the U.S.
The misused/underused acreage is 46.5% of total cropland used for crops. The 335 million acres of cropland is reduced to 179.1 million acres, and would essentially feed the same population, though with substantial changes in diet: little or no meat/poultry, less dairy, much less refined sugar, no cottonseed oil. The U.S. population consumes too much refined sugar, meat/poultry, and total calories, so the population would be healthier and better fed — on 53.5% of the current cropland.
Instead of the usual value of 1.04 acres per person (335 million acres for 322 million persons), the ratio becomes 0.535 acres per person. And this answers the question as to what amount of land is needed for a complete vegan diet: just over half an acre. This calculation assumes that farms grow and people eat the usual complement of foods in this nations, with the exceptions mentioned above. It also assumes that the excess caloric intake typical of the U.S. diet is scaled back to a normal healthy intake, by getting rid of most sugar and meat/poultry calories.
In this scenario, the nation would still grow a small amount of corn and soy (about 5% of current levels). And perhaps we would need to increase our production of grains and legumes, to replace the meat/poultry. But the land reduction would be just as stated above because we can use existing cropland more wisely. For example, we could grow a lot less iceberg lettuce, which is of low nutritional value. We could grow food crops on land used to grow pumpkins for decoration. We could adjust which foods we import and export, based on nutritional needs, rather than mere profit.
Some persons claim that a complete vegan diet takes only 1/6th of an acre (7260 sq. ft. or 675 sq. meters). That is only true with multiple crops per year on the land, no crop failures, and the absolute idea choice of crops and foods. It’s not a practical number than can be applied to a very large population. Based on the above analysis, I’d have to put the land needed for a vegan diet at no less than half an acre per person.