I respect the willingness of vegans to endure hardship for their beliefs. Certainly a vegan is going to have more difficulty staying well fed in a long-term situation.

My understanding is that simple black beans and rice are a fairly complete protein source. Both black beans and rice are cheap in bulk and they store well long term. I have, even though I'm not a vegan, lived for considerable lengths of time on just this. Add a few sliced onions and a bottle of Tabasco and you have a decent meal.

Once cooked it can be reheated or refried for second and any left over can become the base for a soup.

There are other options I'm sure but if the problem can be worked around with so simple an adjustment in diet I don't think vegetarians or vegans would have too much of a problem as long as they prepare ahead and work hard to maintain a balanced diet.

I would add a bottle of multi-vitamins just to round everything out.

Vegans are going to face some issues that us omnivores don't but I don't see any major issues that can't be worked around.