So if you want to be more open about where all the nutritions come from, we should also disclose everything we feed to animals.
Or alternatively, we can simply start enriching some basic foods like plant based milk with b12 and d3.
That said, omnivores often lack nutrients too, including B12. It's seems veganism actually forces you to be conscious about it.
Everybody should be eating a broad diet with a lot of nutrients, including a lot of plants. Vegans have to be more careful about it since they're not getting the broadest array of sources, but I keep seeing this weird trope of "Well, I could eat vegetables, therefore I'm protected against scurvy and constipation even though I never actually eat them."
Both vegan and omnivorous diets are healthy if you eat them well. Both are unhealthy if you practice them badly. But seeking out studies for the sole purpose of proving "veganism is bad" seems like a recipe for eating unhealthily.
In that case, advertisements for meat should also show the bottle of "supplements" that livestocks get (including B12, btw). In that sense, vegans just cut out the middlemen (or better: middleanimal).