they better avoid fruit then too- cos bees pollinate it. also any vegetable grown in soil or with artificial nutrients, as it'll most likely have been involved in the dealings of worms and bugs, and be made with nutrients sourced from the decay of little creatures who have composted and make up the soil. they're also gonna struggle with walking on pavements, living in houses, using tools, etc.
everything is part of everything. what counts most to me, is not doing concious willing harm to creatures who are around now. anyone who died a bajillion years ago is kinda fair game, as far as i'm concerned. i doubt they'll be dreadfully offended or distressed.
most vegans i know who avoid petroleum stuff do so cos they beleive its foul and unhealthy, not cos it might have a bit of cow in it somewhere.