Don't some senior devotees in ISKCON advocate veganism?
The Krishna conscious standard of truth requires harmony between three departments of authority—shastra (scripture), sadhu (saintly persons), and guru (the spiritual master). If all three agree on an issue, we confidently accept their conclusion as true. If shastra doesn't address an issue, we fall back on the statements of guru and sadhu. If guru has nothing to say on a topic, we're left with statements of shastra and sadhu; and sadhus are sadhus by virtue of their words and deeds being consistent with shastra and guru.
Sometimes sadhus, or even respected Vaishnava acharyas, disagree on an issue, in which case we defer to the verdict of guru. Some shastras—such as the Manu-samhita—prescribe courses of action which ISKCON's founding guru, Srila Prabhupada, have dismissed as impractical for this day and age. ISKCON's history shows various attempts to establish new practices not supported by either shastra or guru; initiating women as sannyasinis is one such example. In such cases, the persons establishing the new practices do so without reference to sadhu or shastra, and simply consider themselves gurus with absolute authority.
Sometimes we hear that "the world has changed since Srila Prabhupada's time," to support doctrines and philosophies that Srila Prabhupada himself didn't advocate. The assumption in such cases is often, "if Srila Prabhupada were here now, he would certainly do X, Y or Z in response to the changed world situation."
We've also heard instances of Srila Prabhupada's disciples expressing doubt that he was fully aware of what was going on in the world, or dismissing certain aspects of Srila Prabhupada's presentation of Krishna consciousness as culturally relative; "Srila Prabhupada said X, Y, or Z because he was an elderly Indian gentleman who grew up in early twentieth century colonial Bengal."
Yet anyone familiar with Srila Prabhupada's modus operandi—especially in regard to instructions he wanted to impress upon his followers—knows that his habit was to explain a point very patiently, from top to bottom, back to front, inside and out, until he was satisfied that the recipients thoroughly understood him. This is why he expressed great disappointment that so many of his students didn't read his books, in which he painstakingly explains the entire gamut of Krishna conscious philosophy.
Srila Prabhupada made many statements in his books regarding milk consumption and cow protection. He clearly wanted his readers to take seriously his exhortations to develop self-sufficient, agrarian communities based on cow protection, because of milk being such an important element of a diet supportive of self-realization. He repeatedly condemns the modern slaughterhouse economy as the root of all the world's problems. He plainly expresses pain at the condition of modern industrial farming procedures, and the harm done to cows in the name of fiscal gain.
Absent from his teachings are any indications that he supported abstinence from milk products as a means of bringing about the desired result of a peaceful, Krishna conscious society. He instead consistently takes the position—as he does on a wide range of issues—of "making the best use of a bad bargain;" that all problems can be solved through promotion and application of Krishna conscious principles, individually and collectively. This, he indicated, is accomplished by working sincerely to develop cow protection programs as part of developing local, agrarian economies, and by offering available cows' milk to Krishna.
Since two of the three departments of authority are silent on veganism—shastra doesn't advocate it as a progressive spiritual principle, and neither does guru (Srila Prabhupada is considered the guiding spiritual master of all devotees in ISKCON)—it is difficult to accept veganism on the strength of sadhu alone. Everyone is entitled to their opinion, but personal opinion isn't sufficient for establishing doctrine.