With respect, the substance is no longer bread. The accidents of the bread and wine remain, but as Catholics, we are obligated to believe that in every way that matters it is the body and blood of Christ. Therefore, under the strictest interpretation, it is in fact eating meat. However, under this strict interpretation, it is also cannibalism since Jesus’ body is a human (though transfigured) body.
To address the original question, I would say that just as certain orders of Monks that abstain from meat (for example, the trappists) make an exception for the eucharist so a vegan can. After all the normal objections vegans normally have to eating meat would not apply to communion.
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Bill