I wont claim to know all the facts, but have you factored in the production output per acre, and the required recources per km in the soy V meat? Maybe its simply about picking the LESSER of the evils :tape:
Given that dairy =/= meat, the argument is something of a strawman, but I'll bite anyway:
Soybeans are a pretty environmentally nasty crop no matter which way you spin it. Given dairy is - rather by necessity sourced locally and therefore not going to be responsible for continued land clearing, and particularly not in the tropics.
It's true that energy efficiency decreases as you go up trophic levels (which is why we have trophic pyramids), hence a kilo of meat requires more energy input than a kilo of grain or vegetable matter. However, in terms of nutritional content, they aren't equal. If you look at nutritionally equal vegetarian/vegan and meat meals, There is often a larger land area requirement to produce the vegan option, particularly when compared to intensive meat production and especially where feed for those animals is generated from a substance which is otherwise a discarded byproduct.
This is intensified when you look at organic produce. Organic production is, by default, lower yield per area than intensive production, which means more land area per unit of produce. Added to that is the fact that often this produce is handled in small volumes, which means more transport and process in cost per unit in terms of money and carbon production.
While organic/vegan food is kinder to the domestic animals involved, probably better for you and in many cases tastes better than intensively farmed food, it generally costs more because more resources are required to produce it. This makes it a more environmentally costly way to nourish yourself - it's essentially a first world luxury in a world where half of the 7 billion people on the planet don't get enough food. If we all moved to a diet of lower productivity requiring more land per person to feed us the net environmental outcome is unequivocally negative.
That's not to say that there aren't severe and pertinent environmental issues with meat and intensive farming (rice in the Murray Darling anyone?) but fertilizers, pesticides and transgenic crops aren't always environmentally evil and can, used the right way, be extremely environmentally beneficial.