That wouldnt change anything when it comes to the amount of 'killing' going on. Turn a wheatfield into a forest and you'll have huge massacres going on every day. Plants killing insects, insects killing plants, fungi killing everything and being killed by almost everything.

The only impact humans have in that cycle, really, is changing who gets to kill whom. If you weren't feeding soy to cattle, than the space of the soy field would be occupied by wild cats killing squirrels.

The only thing a vegan/vegetarian is reducing is the variety of his/her diet.