If the whole world went vegan overnight, the consequences would of course be unpredictable. Suddenly, we would have a high demand for medical methods to prevent the deer and moose population from exploding. Then we would have an awful lot of unemployed people and unused infrastructure from livestock facilities. Same thing with milk and egg production. Since everyone would be vegan, current dairy cows or chickens could not be killed, so an incredible amount of funding would be needed for animal sanctuaries. In addition, a very small part of the vaccines are vegan, so who would be developing those if everyone was vegan. We would also need many more landfills because wool, leather and other animal products would go into the trashbin.
So, hypothetically, people living in the tundra would be our least problem, because the entire current infrastructure would collapse and there would be a rather extensive crisis going on in pretty much all developed countries.
If we prepared to go vegan with a 30-year plan (realistically, much longer), that would be a different matter, but it would still require years of planning work by experts.