Warning CC.....
I am not an ethical vegan nor a bible scholar, but man did not eat meat until after the fall. I believe God told Noah to eat meat, but before that all the seeds of the earth were for their meat. I can't answer the B-12 thing and how we need it and why it isn't in plants, but I just don't believe we were created to eat meat.
Gen 1:29 And God said, Behold, I have given you every herb bearing seed, which [is] upon the face of all the earth, and every tree, in the which [is] the fruit of a tree yielding seed; to you it shall be for meat.
Gen 1:30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein [there is] life, [i have given] every green herb for meat: and it was so.
Gen 9:1 And God blessed Noah and his sons.....
Gen 9:2 And the fear of you and the dread of you shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth [upon] the earth, and upon all the fishes of the sea; into your hand are they delivered.
Gen 9:3 Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.