It's not healthy to only eat vegan food without monitoring and supplementing vitamin B12. A nutritional vitamin B12 deficit can go unnoticed for years - when it is noticed damage has already been done. Humans are omnivores that need animal food sources from time to time (although it does not have to be meat).
Also I don't understand vegan ideology. Why should it be any more acceptable to slaughter plants than animals? Just because animals are more similar to humans than plants and evoke feelings of pity that plants do not? Plants are also living creatures! Assuming that slaughtering plants is acceptable while slaughtering animals is not is therefore a prime example of ideology-based Darwinism.
ya.ya.yo!
The statement about B12 is inexact. All of the B12 in nature is created by micro-organisms, and one does not need to eat meat for that - although I'll admit that all research around that vitamin is quite hazy since it's a difficult field of study. There's evidence, although not 100% sure, that you can get enough of it by eating fermented foods like sauerkraut and vegan yogourt, and it might even be found in the algae used in sushis. There's also theories that affirm that anyone with a healthy gut has enough good micro-organisms inside of him to produce his daily required amount of B12 on his own. I do agree with you that its important to make sure we have enough of it by being aware of the early symptoms of deffiency (mostly, being often tired).
Besides, most B12 that is found in meat is artificially added to the animals through supplements fed to them, since they do not produce enough on their own when fed with the inadequate food typical of factory farming. So being a vegan that gets artificial B12 through soy milk or whatever is not different.
It is more acceptable to "slaughter" plant, since they don't feel pain.