@barn Thanks for welcome.
However, in this case, it's much simpler than your post pre-supposes. The US is already operating as very near 100% capacity in terms of vegetative foodstocks. Most people don't know this, I do because I looked it up one day while creating a mathematical proof for bio-fuel production. However, it is true if you look up the volume of farmland and then deduct the acreages of land which farmers are paid, using tax revenues, to not grow on.
In spite of the fact that the farmland, the literal available area on which to grow foodstocks, already operating at near capacity: the US only produces a surplus in vegetative food stocks in the form of corn to the tune of 1.631 billion bushels left unsold.
Veganism, unlike simple vegetarianism allows for no animal products and no animal byproducts. No red meat, no poultry, no fish, milk, cheese, butter, etc. etc.
There is not a volume of land mass on which to grow vegetation in a vast enough quantity to support a substantial portion of the US population even if you combined the vegetative growing territories of several countries not just the US. Even employing the extremely advanced agricultural system which is already in place today which does make use of poly-cultures, mono-cultures, genetically modified seeds, chemically fertilized fields, multi-harvested crop rotations.
It's theoretically possible to create self sustaining societies in a far off Logans Run dome city complex by building multi tired vegetative food stock groweries. Places with thin soil which is constantly saturated with chemical fertilizer through sprinkler systems and the plants are bombarded with UV light nonstop in a temperature controlled area, thus radically reducing the time between harvests with no risk of soil depletion. By taking crop production in doors and applying what methodologies pot growers have developed in the realm industrial herbaceous plant production: you have the potential to increase vegetative food stock production.
I say in "theoretically": because this has never been done - and therefore the absolute numbers on what plants can be grown in such a way for what level and size of facility, along with just how often said plants can be harvested in this manner, are all unknowns as of yet. Theoretically, because you should be able to grow things like yams, potatoes, beets, cabbage, carrots, etc. with fairly shallow soil: you could grow foodstocks on tables which have drainage run offs to get rid of excess chemical fertilizer and UV emitting lights bombarding the tables at all times. That should allow said plants to reach harvest size faster than in fields.
-However, another issue which is as of yet unknown: will the vegetable in question have the same nutritional value if it grows quite as rapidly?
-Also, this methodology only functions, in an area efficient manners with shallow soil or top soil plants.
Much of this discussion, though relevant to talk about: has breached reality into the realm of science fiction (currently). I don't have an issue with talking about theoretical possibilities: but I strongly urge against abandoning the realm of reality in order to chase theories on "What could be done at some point." Just because I'm smart enough to design a foodstock production complex which would dramatically increase food production capacity: doesn't mean I have the funds to build it and it also doesn't mean anyone else is going to be build it anytime soon. The future will come in it's own due time, the present is what's most pressing.
At present: there is not a way to support a vegan diet for a substantial portion of the population of the first world - which is the only society technologically capable of supporting a vegan diet without an individual becoming malnourished. Which excludes the vast majority of the population on the planet. At present, only a fraction of a fraction of the human race is capable of living on a vegan diet due to the technological infrastructure required to enable such a diet (industrious agriculture + artificially produced chemically purified nutritional supplements).
This is why the vegan argument "needlessly killing animals is wrong, we should all be vegan" is a complete and utter fallacy.
Now... All that being said.
If someone wants to argue that people consider veganism for health benefits: confirmed - that's a good argument.
If someone wants to argue that people consider veganism for weight loss: confirmed - that's a good argument.
It is only the emotionalist argument that everyone should be vegan because those poor animals which is fallacious because it's a pipe dream.
I don't mean to say pipe dream as in communist Utopianism: I mean fallacy as in asking that two plus two equal five.
In this case, it's asking that landmass Q, physically capable of producing P, instead produce R which is a dozen magnitudes greater than P.
Two plus two equals four, not four-hundred, which is what is being asked.