It is just a matter of fact that eating soybeans in such a big amount is destroying the rainforest. There’s nothing more to be said here.
It is a matter of fact about the current agricultural practice, debatably. Vegetarianism, however, is not about the current agricultural practice. It is a general position that requires counterfactual reasoning. And what I am saying if everyone who is a vegetarian now starts to eat meat, then it would be destroying the rainforest even quicker (because it would take even more land to produce one calorie of meat than one calorie of soy -- those cows have to eat something). Conversely, if everyone who eats meat now converts to vegetarianism, then the land we are currently using to produce food for cows can be used to plant, say, soy. Consequently, we wouldn't need to deforest the rainforest. By the way, there are monoculture pasture farming of cows that are destroying the rainforest too, and that is killing the land because the land loses all its soil after about 10 years.*
Vegetarians defend V2. V2 does not have the consequence you think it has. Namely, V2 does not imply deforestation. Just because some stuff that some vegetarians eat are produced as a result of deforestation does not mean that vegetarianism endorces deforestation. In fact, I would say the practical action would be to plant vegetables where we currently plant corn (which is what we feed to cows). Then with the same space, we produce more calories.
It follows from V3 and the actuality that vegetables are lower than the animals we eat (except maybe snails) on the food chain that we should eat vegetables only. But good, I need to clarify this: V3 is a moral claim, but it is not a claim motivated by moral concerns about animal suffering. The latter is what I call the moral version of vegetarianism, short-handedly and perhaps confusingly.
* reference: "Industrial agriculture and beef production for example, is a major cause of deforestation in the Amazon, to raise cattle. This is not even for local needs, but to meet fast food restaurant demands in the Northern countries."