One of the many arguments against veganism revolves around the idea that, for one reason or another, not everybody can subsist solely on a plant-based diet. Although very rare, it is indeed true that some people are (not necessarily permanently) unable to survive without animal products. This, however, doesn't necessarily mean they're not vegan.
Veganism isn't about being perfect. We all make mistakes, and since we live in a non-vegan world, it's near-impossible to completely avoid contributing to suffering in some way. This, however, doesn't mean that veganism is impossible. As long as somebody tries their absolute best to live a compassionate lifestyle and avoids contributing to harm as much as humanly possible, they're vegan.
"Veganism isn't about being perfect. We all make mistakes, and since we live in a non-vegan world, it's near-impossible to completely avoid contributing to suffering in some way. This, however, doesn't mean that veganism is impossible."
This sentiment remains true for people living in poverty. In dire times where people have absolutely no choice in what they eat - if they have to live off of scraps, for example - they may have to unwillingly consume animal products. This doesn't make them any less compassionate - or any less vegan, for that matter - they are only doing what they need to do to survive, as is the case with most of Earth's creatures.
This isn't a matter of convenience, it's a matter of necessity. If you choose to eat plant-based unless it is inconvenient to do so, you're not vegan. This is a selfish act indicating that personal convenience is more important to you than the lives of those who are suffering. If you think shopping ethically is only important when it's easy for you, you're not vegan. Sometimes, if you're aiming for a better world, small sacrifices are necessary.
Most people do have a choice in what they eat - and, if compassion is their priority - they will choose not to consume or use animal products. Even if supermarkets are not readily available, vegan food can be grown in large amounts in your own backyard for a low cost - plus it's sustainable, self-replicating and incredibly healthy! Vegan food consistently replenishes itself, and it doesn't need to be fed other food!
Most people without much money subsist on plant-based diets as it is, since plant-based food is some of the cheapest and most easily accessible food on the planet. Rice, beans, lentils, vegetables and grains can all be (quite easily) grown from home, or purchased in bulk from a supermarket for a low price. When cooking (or growing) from scratch, a vegan diet is infinitely more affordable than the traditional Western diet, which consists largely of animal products and processed foods.
If animals are able to survive in a dire environment, it's highly likely that humans will be able to as well. They could share whatever the animals are surviving off of rather than raising them for food. Doing so requires the use of many vital resources such as food, shelter and water - all of which could instead be directly consumed by humans.
In rare cases, however, there simply isn't a choice. People living in urban areas in poverty, for example, may not have access to the conditions needed to grow plant-food, and may instead have to subsist solely through scavenging or through receiving donations from others. This is a matter of survival rather than a matter of convenience. If it's a live-or-die scenario, they are still vegan, so long as they still do their very best to fight for the animals.
"[Some people] may not have access to the conditions needed to grow plant-food, and may instead have to subsist solely through scavenging or through receiving donations from others. This is a matter of survival rather than a matter of convenience. If it's a live-or-die scenario, they are still vegan, so long as they still do their very best to fight for the animals."
If we lived by the idea that somebody is automatically 'un-vegan' as soon as they consume an animal product, whether accidentally or out of complete necessity, our cause would never fare well. Veganism has never been about attaining absolute perfection, nor is it about climbing the moral hierarchy. It's about caring enough to do whatever you can to stop unnecessary suffering. If you live with this goal in mind and do your best to uphold it, you are vegan. If you slip up at any time, or absolutely need to consume an animal product for one reason or another, don't feel guilty. You're not perfect. Perfection is unachievable - animal liberation isn't.