Fact: Buddha ate meat. He died from food poisoning eating pork. Are not thousands if not millions of living being slaughtered when you harvest vegetables (worms, insects, micro-organisms etc.)? So how is vegetarianism morally superior to eating meat in a Buddhist context?
First of all, the Buddha didn't die from eating pork at least in the Theravada point of view. In fact, his last meal was sukara maddava, translated to English dug by pigs, a mushroom which still grows in India.
Also, vegetarianism is not considered morally superior to eating meat in Theravada. The general guideline followed with regards to eating meat by both monastics and laity is,
I say that there are three occasions in which meat may be eaten: when it is not seen, not heard, and not suspected, that the living being has been killed for sake of the bhikkhu, I say: Meat may be eaten on these three occasions.
On the other hand, a person can be a strict vegetarian while craving for tastier food and could be strengthening his ego considering his higher morality, both of which are not good.
It is not clear if The Buddha died from eating pork or mushrooms (sukara-maddava). But it was obviously intentional or planned out (since according to the sutta no one else in the universe would be able to eat such a thing and entirely digest it).
It is clear that The Buddha did not support vegetarianism. He opposed directly killing animals (hunting, fishing, etc...) but eating meat that wasn't directly killed by you or naturally died is allowed. He also opposed damaging plant and seed life.
He also pointed out that ascetic practices (which includes avoiding eating meat, drinking alcohol, etc...) are painful in the present and yield pain in the future (MN 45), they lead to hell or lower destinations as explained because they simply torment one's body without causing one to achieve arahantship or higher states (jhanas).
Vegetarianism fits into the category of an ascetic practice that by itself doesn't cause one to achieve arahantship or higher states (jhanas).
A vegetarian who hasn't ended their mental fermentations is still a non-arahant.
A vegetarian who does evil deeds is still an evil doer.
Practices are only helpful if they lead towards arahantship (the ending of mental fermentations).
"I praise the right method of a householder or of one gone forth homeless. Whether a householder or one gone forth homeless, if fallen to the right method it is possible that he should be convinced of noble merit. " (MN 99)
"Not by shaven head does a man who is indisciplined and untruthful become a monk. How can he who is full of desire and greed be a monk?" (Dhammapada, 264)
"Just as kusa grass wrongly handled cuts the hand, even so, a recluse's life wrongly lived drags one to hell" (Dhammapada, 311)
How important is vegetarianism by itself to achieving arahantship? Not very.
Lots of things you do can indirectly kill living beings besides eating meat (not directly killed by you or your request), better to focus on the things that matter that lead towards arahantship.
For reference, here's what it says in MN 45:
"And what is the taking on of a practice that is painful in the present and yields pain in the future?
"There is the case where someone is a cloth-less ascetic, rejecting conventions, licking his hands, not coming when asked, not staying when asked. He doesn't consent to food brought to him or food dedicated to him or to an invitation to a meal. He accepts nothing from the mouth of a pot or from the mouth of a bowl. He accepts nothing from across a stick, across a pestle, from two eating together, from a pregnant woman, from a nursing woman, from a woman lying with a man, from a food collection, from where a dog is waiting or flies are buzzing. He takes no fish or meat. He drinks no liquor, wine, or fermented drink. He limits himself to one house & one morsel a day, or two houses & two morsels... seven houses & seven morsels. He lives on one saucerful a day, two... seven saucerfuls a day. He takes food once a day, once every two days... once every seven days, and so on up to a fortnight, devoted to regulating his intake of food. He is an eater of greens, millet, wild rice, hide-parings, moss, rice bran, rice-scum, sesame flour, grass, or cow dung. He lives on forest roots & berries. He feeds on fallen fruits. He wears hemp, canvas, shrouds, refuse rags, tree bark, antelope hide, strips of antelope hide, kusa-grass garments, bark garments, wood-shaving garments, head-hair garments, animal wool, owl's wings. He is a hair-&-beard puller, one devoted to the practice of pulling out his hair & beard. He is a stander, one who rejects seats. He is a hands-around-the-knees sitter, one devoted to the exertion of sitting with his hands around his knees. He is a spike-mattresser, one who makes his bed on a bed of spikes. He is a third-time-in-the-evening bather, one who stays devoted to the practice of bathing in water. Thus in a variety of ways he stays devoted to the practice of tormenting & afflicting the body. With the break-up of the body, after death, he goes to a bad bourn, destitution, the realm of the hungry shades, hell." (Cula-dhammasamadana Sutta, MN 45)
Also in MN 12 The Buddha explains how following a certain diet didn't help him achieve enlightenment:
"Sariputta, there are certain recluses and brahmans whose doctrine and view is this: 'Purification comes about through food.' They say: 'Let us live on beans'... 'Let us live on sesamum'... 'Let us live on rice,' and they eat rice, they eat rice powder, they drink rice water, and they make various kinds of rice concoctions. Now I recall having eaten a single rice grain a day. Sariputta, you may think that the rice grain was bigger at that time, yet you should not regard it so: the rice grain was then at most the same size as now. Through feeding on a single rice grain a day, my body reached a state of extreme emaciation. Because of eating so little... the hair, rotted at its roots, fell from my body as I rubbed."
"Yet, Sariputta, by such conduct, by such practice, by such performance of austerities, I did not attain any superhuman states, any distinction in knowledge and vision worthy of the noble ones. Why was that? Because I did not attain that noble wisdom which when attained is noble and emancipating and leads the one who practices in accordance with it to the complete destruction of suffering." (Maha-sihanada Sutta, MN 12)
Also The Buddha says:
"Monks, one who hasn't abandoned nine things is incapable of realizing arahantship. Which nine? Passion, aversion, delusion, anger, resentment, arrogance, insolence, envy, & stinginess. One who hasn't abandoned these nine things is incapable of realizing arahantship." (AN 9.62)
This means those who abstain from eating meat and follow other rules but haven't abandoned these nine things would still be incapable of becoming arahants even though they follow all types of rules and ascetic practices.
Not a direct answer just some points about this :
1 - More crops are grown specifically to feed livestock than to feed humans
so the move of people to a vegan diet will reduce the amount of crops needed
(and yes animals are fed non-edible plant parts and feces "fish flour" ect but thats on top of the crops you grow - about "grass fed" first its a tiny tiny part of the meat in genral and even grass fed are fed crops- by piles of crops put in the fields - by feeding cattle in winter time - by feeding the cows a high calorie diet some months before slaughter ect)
so if someone really cares for insects ect that's just another good reason to become vegan !
2 . A good source on the buddhas diet is : http://www.dhammawiki.com/index.php?title=Diet_of_Buddha
3 . A good video about eating meat is "animals and the buddha" :
4 . This is hard to explain in non-buddhist sites but here its simple : INTENTION to kill is key - a lot of the deaths in crops are not intended to be dont (some are but not all) unlike the deaths in the meat industry which are intended - so when you buy meat you pay for someone to kill you pay someone to cause some very bad karma
So INTENTIONAL KILLING vs UNINTENDED KILLINGS ..... and knowing for sure that when you buy meat its going to cause intentional killing
and yes its not like killing the animal yourself ... but still there is a difference - and this is another thing people should take into account
5 . i follow theravada buddhism so i talk from this point but i know there are vegetarian sects in mahayana buddhism - but maybe im wrong didnt research this - but my view in theravada is you can eat meat - but it is conducive to avoid eating meat completely and being vegan
6 . monks and lay people are very different and theres a different between a monk accepting meat and a lay person buying meat at the supermarket