If I were a vegan and starving, I might easily find myself eating any meat available. However, I also think vegans might have trouble if their diet changes quickly. Moreover, it's their choice to eat meat or not, IMO. If they choose to starve, that is their choice. They'll know it when faced with it.

I think the difficulty for a vegan if they are faced with a need to change is that they might not be prepared to do so. I have a few things in my BOB for catching animals, should I ever need to. Kind of tough to test out a snare trap in NYC, and your choice of prey is limited to mostly things I do not want to eat. If push comes to shove, we'll see if I am willing to choose to change my diet to strangled rat.

Nevertheless, a vegan's best bet is to prepare and store some food. I might suggest a good store of dried fruits and vegetables, like: http://www.harmonyhousefoods.com/mainhome.html.
The TVP would work for protein. Rice stored can last almost indefinitely, if not cooked or found by other life forms. Some things that would allow some hunting would also be a good idea, if the vegan should decide otherwise when desperate, but your opinion may differ.