Being vegan is not a private or personal choice. To some it is a threat, no matter how quiet you are about it. Even just sitting at a table and eating a salad (not that a salad is ever an acceptable alternative to a steak) raises questions. Why is she or he eating that? Should I be eating that? Am I being judged because of what I'm eating? I've found that many of these questions go unasked. Without a successful outlet omnivores often lash out in inappropriate ways at vegans, which can sometimes make sense when we consider the back log of unanswered questions they've been stewing in. Seeing a vegan as a self righteous and self imposed judge is a lot easier than seeing yourself as a murderer. When successful outlets exist, usually a positive relationship based on sharing between vegans and omnivores, each question is answered one at a time. Taking the questions one at a time can show us that they aren't about the character of the person who is eating, but really about the non-human animal that is being eaten or used in some way. It is important to deal with these questions as they arise, but since that isn't always possible we need to figure out how to deal with that outburst that happens later. About half of being vegan, and an integral part of Post-Veganism, is about living by example and opening up discussion when possible.
A typical outburst, and one I find extremely unhelpful, is one person initiating a 'what if?' scenario. I've heard it many times. What if the cow was free range? What if the eggs were laid by happy chickens owned by this lady I know from work? What if a native person hunted the non-human animal from the forest? These are all important questions and should all be answered, but only when appropriate. Most of the people, I think all of them actually, were omnivores who regularly bought flesh, chicken eggs and cow milk from the grocery store (which means they consciously or subconsciously support factory farming). None of these questions apply to their life in any way, they are only interested in the theoretical.
These questions, from what I gather, have a dual purpose. Firstly, they are questioning your vegan-ness. They are assuming there is a situation that exists where it is acceptable to not be vegan. In assuming this they believe you have either overlooked it (and upon realizing it won't be vegan anymore) or have considered it and are still vegan to keep up appearances (but there is no real moral justification behind it). Secondly, they are looking for a way it is acceptable for them to not be vegan. In doing so they have already decided, for whatever reason, that they don't want to be vegan.
When confronted with the 'what if' scenario the best thing I think anyone can do is just not answer the question. What if a native person hunted their own food with a bow and arrow is very different from the person before me who buys into a culture where 'food' doesn't have a chance to run away because it's never even seen the sunlight. Even if I said a native person was morally right to hunt their own food (which I don't believe is true, every life is equal and since it is not right to kill or harm a human it is not right to kill or harm non-human animals) it doesn't mean this person will go hunt their own food. They will take up time trying to weave a scenario where vegan does not work, and when they finally do do it (if you let them) this person will live in the nostalgia of the mind. They are happy just to know that it is acceptable in some way, even if what they do doesn't at all remotely resemble the acceptable way.
I doubt that any person does this with the purpose to deceive or trick a vegan. I think it is a subliminal reaction, a flight response in a tricky situation. When 'what if' comes up - I think the best thing to do is steer the conversation to the present. A response could be 'well that's an interesting idea, but it doesn't apply to your life does it? Do you hunt your own non-human animals? How about we talk about where your non-human animals come from?'
Be careful with 'could be', a relative of 'what if'. This could have egg in it, but that doesn't mean it does. The non-human animal could be from a slaughter house, but that doesn't mean it is. Could be exists for the same reason as what if, to derail the conversation and find an acceptable safe territory. My response to could be is always 'look into it'. Based on our society's cultural reliance on the carcasses of abused other animals it is more likely that it does contain non-human animal products from slaughter houses. Even if that were not true and it were only a 50 50 chance to contain or not to contain, the person is consuming on the assumption that it is not so. If it was so, would they still do it? That is the real question that should be asked. If they would still consume it if it did contain it or it was true, saying could be is just a way to cover up the truth. If they wouldn't consume it than they should look into it, because some moral dilemma is clearly stopping them from doing so and they wouldn't want to consume something if it did contain the adverse product.
As a final note I would like to say that it is important to live up to what you say. I remember once saying to a friend something like 'well I guess it would be different for someone who is anemic and is told they need to eat meat by doctors, but that's not the situation here' only to find out that this person was in fact anemic. We shouldn't avoid saying this in case it backfires (because it rarely will), but be ready to talk about it when it is relevant. It was a difficult conversation to have, but it is a very different conversation to talk to someone who is physically anemic than someone who is hypothetically anemic. Upon telling this person about how cow milk is made she was instantly horrified and upset. The first thing she said was 'I had no idea' and then asked for more information. While it was difficult for us to talk about meat (as she had tried being vegetarian and her doctor told her she couldn't be anymore), talking about products like cow milk and chicken eggs that she doesn't need to consume (which cause just as much suffering as flesh does) was easy because she wasn't asking 'what if' to escape a moral dilemma.