Let’s compare the amount of agricultural land used to produce plant-source foods (vegan food) versus animal source foods (meat, poultry, dairy). The comparison is enlightening.
According to the USDA ERS , the United States uses the following areas of agricultural land to feed its population:
335 million acres as cropland for crops
36 million acres as cropland for pasture
37 million acres as cropland left idle
614 million acres as grassland pasture and range
So at first glance, it looks like 614 plus 36 million acres, 650 million acres total, are used for livestock pasture and range, versus 335 million acres for other food crops. But those 335 million acres of cropland for crops includes the land used to grow corn for ethanol (biofuel), 34 million acres, and corn for livestock feed, about the same acreage.  So if we wish to compare land growing vegan foods versus land growing meat/dairy, we need to adjust the numbers:
335 million acres of cropland, minus 34 million for corn/ethanol, minus another 34 million for corn/livestock feed, equals 267 million acres of cropland for crops. I would also deduct 11 million acres used to grow cotton. We get very little cottonseed oil per acre from these crops, and it is of low food value. So now we are down to 256 million acres of land used to grow plant-source foods.
Soybeans are also a major component of livestock feed. The U.S. uses 36 million acres of land to grow soybeans for livestock feed, and another 36 million acres to grow soybeans for export.  But the exported soy is largely used just as in the U.S., for livestock feed. On the other hand, we do obtain much of our vegetable oil from the same soybeans used as livestock feed. The one crop provides two products: feed and oil. Since this land provides two major products, one plant-source food (soybean oil) and also livestock feed for animal-source foods, let’s be fair and divide the land in half, 36 million for plant source food and 36 million for the animal source side of the calculation.
That leaves us with 220 million acres to provide a vegan diet for the U.S. population. We are, in this hypothetical, taking away animal source foods, which provide fat and protein. But much of the cropland in the U.S. is used to grow food that is exported, or is used inefficiently (e.g. sugar beet and sugarcane, which provide only empty calories). So we would not be shortchanging this side of the calculation if we leave the total at 220 million acres for our population of 322 million persons (0.68 acres/person). A more judicious choice of which crops to go would make that amount of land sufficient.
For animal-source foods, we have 614 million acres of grassland and range, plus 36 million acres of cropland for pasture, plus the 34 million acres of corn for livestock feed plus half of the acres of soy for livestock feed, which is another 36 million acres. The total is 720 million acres of land used for animal source foods.
220 million acres is sufficient to supply the entire U.S. population with a vegan diet. And additional 720 million acres is used to provide us with animal-source foods — more than 3 times as much land. If those 720 million acres were also used to provide a vegan diet, the agricultural land in the U.S. could feed the entire U.S. population PLUS another one billion persons. And we would still have an extra 37 million acres of cropland left idle (from the above USDA numbers) to use as we saw fit.