This scientist, Steve Davis, concludes that if we're trying to kill as few animals as possible, we'll do better to eat beef--as long as it's fed on grass--then to follow a vegan diet.
The economist, Gaverick Matheny, took him to task for it about a year later in the
Journal of Agricultural and Environmental Ethics:
http://jgmatheny.org/matheny 2003.pdf
You should read the whole thing because he makes some other good points. But, basically, Davis made a gross error in his calculations. He assumes that an acre of land will feed the same number of people irrespective of whether it's used to raise grass-fed beef or to grow crops. In fact, an acre of land used for crops will feed about ten times as many people as an acre of land used for grass-fed beef. When that difference is fed into the calculations, Davis's argument is turned on its head, and proves that vegans are indirectly responsible for killing only about a fifth as many animals as those who eat grass-fed beef.
Davis is also only considering the number of animals killed and not the suffering they endured while alive.
"Early in the paper, Davis shifts from discussing the harm done to animals under different agricultural systems to the number of animals killed. This shift is not explained by Davis and is not justified by the most common moral views, all of which recognize harms other than death... Davis, in discussing the number of animals killed rather than their treatment prior to death, ignores an important question that must be answered in order to assess which system of agriculture causes the least harm."
"Predictably, his argument has been cited as a justification for traditional omnivorism, a misreading Davis did not intend and one that any faithful reading of his paper should prevent."
So, yeah, eating vegan kills some animals. But not as many as does meat-eating, and without the numerous concentration camps for animals.
If the argument is least harm and suffering, then vegans have the ethical edge over meat-eaters, both in numbers killed and in treatment. But this shouldn't be about moral superiority, just about doing the right thing and trying to cause the least harm if there's no way to avoid it.