This is silly. The solution to a vitamin deficiency is vitamins. Clearly they're deficient while eating meat too, so that doesn't seem like a strong argument against plant based proteins.
No, they are not "clearly deficient while eating meat" as you claim. There's a documented B12 deficiency in vegans and vegetarians that isn't present in meat eaters.
Cherry picking one vitamin deficiency in vegans. Omnivores are deficient in other vitamins. The answer is supplementing, not changing foods you eat.
> Omnivores had the lowest intake of Mg, vitamin C, vitamin E, niacin and folic acid. Vegans reported low intakes of Ca and a marginal consumption of the vitamins D and B12.
[0] http://www.epic-oxford.org/publications/1554/serum-concentra...
No, they are not "clearly deficient while eating meat" as you claim. There's a documented B12 deficiency in vegans and vegetarians that isn't present in meat eaters.