no, it's a known fact that the slash and burn style of farming in the region is the cause of the deforestation, they burn the field, completely drain it of any nutrients to grow the soya with absolutely no thought to sustainability because setting a fire is far cheaper, therefore far more profitable, than sustaining land you've already cleared. If they stop growing cows for beef, can you guess what will happen? If not, I'll tell you, they'll burn down part of the forest to clear it, grow soya until it's drained the land of all it's nutrients, and then burn down some more forest to clear it for more soya, which will then proceed to drain that bit of land of all it's nutrients. When that's done, they will burn down some more forest, grow more soya until it's drained the cleared land of it's nutrients etc etc.
Going vegan will have changed absolutley nothing.