This is solved by a trivial supplement. Millions of vegans/vegetarians take them every day, and many hundreds of millions of livestock (which is how most get their B12, as well). If we didn't supplement our livestock with B12, we'd all always be deficient.
A lot of suffering is avoided with a measly supplement.
In the case of B12, yes. B12 deficiencies are a result of modern farming practices, _not_ plant-based diets. This is why our livestock are supplemented with B12.
B12 was originally sourced naturally by consuming things straight from the ground, like foraging and drinking from streams, or eating animals. Modern farming doesn't allow for a lot of this consuming straight from the wilderness though, so B12 needs to be supplemented. Either into animals so we get it via their meat, or directly, via a supplement.
Your B12 is coming from a supplement either way, though. Vegans just ask that you take out the middleman so that they don't need to die for you to get something that is found in a measly pill.
A lot of suffering is avoided with a measly supplement.