i agree that one can't be "100% vegan." But, i believe that one should strive to do the least harm possible with what they're capable of doing. Perhaps some people only want to eat certain meat and dairy products. Perhaps others can forgo all animal products in their foods, but not in other places. Perhaps others can become "like the amish." it just depends upon the individual, the choices that there making.
for me, the real turnkey is not what the person is doing, but the conscienciousness with which one does somehting. So, not the what, but the why. One can 'be vegan" in conscienciousness, and not be "as vegan" as the next vegan. (like the one who may "be like the amish")
similarly, i don't think in terms of "sin" or even "karma" for that matter. Actually, we're in a bhakti (or devotional) age, so the wages of sin (the effect of karma) is nothing, if we have bhakti (or so Jesus, et al., said.
). Again, we move from the idea of WHAT one is doing or has done to WHY one does it. This makes "being vegan" so much easier than trying to cross out as much karma as possible. As many texts have said (and i'm thinking of a particular upanishad that i can't recall the title of right now), doesn't matter what you do, as long as your heart is focused on righteousness, no harm can come to you--because action and inaction are one, when one is devoted to God and truth (ah! the bhagavad gita!). Ultimately, karma is irrelevant, the actions and inactions (the consequences there in) are merely more tools for learning and developing devotion.
So, eating ice cream is not a sin, depending upon WHY one eats ice cream. or WHY one doesn't eat ice cream.