How vast? We're basically talking of the same land - how many cows it could support directly vs. indirectly via feedstock production. I don't see any reason why it should be many times more.
Feedstock production is highly efficient. Grazing animals for meat is very inefficient in terms of caloric output. This is to say nothing about morality or even nutrition.
Perhaps, in ideal world, nobody would eat meat. But that isn't going to happen. And zealous painting any and all livestock farming as immoral and evil isn't likely to get us there.