Ive been vegan for more than a decade and most of my circle is vegan. I don't think i know of any vegans against cut flowers. The only argument i can see is that if you were cutting flowers found in the wild then you could be harming the environment which indirectly hurts the wildlife and pollinators. But i think most vegans would be ok with buying flowers. My wife and i also just grow our own and cut only a fraction of what we plant.

Btw if we're talking about Costco for chicken, they do have reasonably sized bouquets for as cheap as $10. And while that chicken will be gone in a day or two, Ive kept a Costco bouquet alive for my wife for a minimum of a week.