On my Cambridge Pork Pie event at the weekend, everything at arrivée was intentionally vegetarian, and the (homemade) soup and stew were vegan and gluten-free.
I had one declared vegan on my 200 which prompted me to adapt the recipe for the flapjacks I was providing at the start to use Trex instead of butter. I don't think anyone even noticed - certainly didn't get any complaints, far from it. I had considered doing two batches, one with butter, one with Trex, but then I decided that was too much faff. Nutritionally, there's nothing missing from a vegan flapjack - in fact, because I also added fresh fruit (apples) to the recipe, they were probably more nutritious than most.
I also made some pastries using shop-bought puff pastry, which may or may not be vegan - ingredients list only vegetable fats, but I wouldn't trust them not to contain traces of dairy.
Also made sure I included a proper vegan option in the food at the finish too. I don't think it hurts organisers to cater for the increasing numbers of vegan cyclists, and it's no hardship for non-vegans to go without meat or dairy occasionally, so if I were doing the catering at the finish myself, I would probably make all the food vegan.
What's more, meat and dairy products are expensive, so vegan food is often cheaper, which is an important consideration for audax organisers on a limited budget (my flapjacks were about 40% cheaper than if I'd used butter).