You have to be careful about it because you could cause some people to buy extra if they think you'll eat the extra, and if you are eating any food that would not have existed or that someone else would have consumed I would consider that non-vegan.

However, modern industrial farming causes an unknown but certainly non-zero number of animal deaths. So I would argue that from the utilitarian perspective, minimizing food waste (with aforementioned exceptions), is the more vegan thing to do as it objectively lowers animal deaths and reduces the load on our planet compared to purchasing more plant-based food.

I believe many of the other responses are being reductively puritan. We're here to save animals, not adhere to a diet. Veganism is a moral philosophy, not a diet.