Do you know if there are any studies on the incidence of peanut allergies in vegan families vs. non-vegan? Vegans would exclusively use soy formulas, and they pretty much live on soy anyway, don't they? I see all those soy burgers, soy dogs, soy cheese (yuck and double-yuck!)at the grocery. I'm assuming pregnant vegan women would continue eating it, and that soy (or whatever the allergen in it is) might cross the placental barrier as well.
I checked the medical literature and did not find anything on vegans and increased allergies; on the contrary, the literature has a few studies suggesting that vegan diets can help in some autoimmune disorders such as rheumatoid arthritis. And remember, it would only be the early sensitization with soy formula that would likely make the difference.